7.5 Technical Descriptions and Definitions

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Descriptive technical writing uses a combination of visuals and text to both “show” and “tell” the reader about the information being conveyed. Like more creative descriptions, technical descriptions sometimes draw on the “five senses” and metaphorical comparisons (analogies) to allow the reader to fully conceptualize what is being described. More often, however, they rely on concrete, measurable descriptors. Technical descriptions can take many forms, depending on purpose and audience. Descriptions can range from a brief sentence, to a paragraph, a whole section of a report, or an entire manual.  Poorly written technical descriptions can cause confusion, waste time, and even result in catastrophe!  Technical product descriptions are often legally required to ensure safety and compliance. Attention to detail is critical.

Product specifications require detailed descriptions of design features; instructions often require specific descriptive detail to “show” the reader what to do. Some general categories of technical descriptions include the following:

  • Mechanism Descriptions: Provide a detailed overview of the physical aspects of a tool, machine or other mechanical device that has moving parts and is designed to perform a specific function. These could be product descriptions for sales or manufacturing, documentation of design specifications, infographics, etc.  This chapter focuses in detail on this kind of description.
  • Process Descriptions:  Detail a series of events (natural/biological/ecological, mechanical, social, or psychological phenomenon) that happen in a particular sequence in order to achieve a specific outcome. These can be categorized into non-instructional processes (such as a process that analyses how an internal combustion engine works, or natural processes like photosynthesis) and instructional processes (such as recommended/required procedures and explicit step-by-step instructions to be followed). (See Section 7.7 for detailed information on Writing Instructions).
  • Definitions:  Clarify the specific meaning, often related to a specific context, or express the essential nature of the terms being defined. These can range in length from a simple clarifying phrase to a glossary, to an extended document of several pages. Definitions will often include detailed descriptions and visuals to illustrate ideas. Extended definitions are created using various rhetorical strategies: classification, exemplification, analogy and comparison, history, and components, along with relevant visuals. You can find a good example of such a description on the NASA Mars Perseverance Rover site; the overview of the spacecraft that conveyed the Perseverance Rover to Mars includes various rhetorical strategies. Can you pick them out? Click on the link below to view a student PowerPoint presentation on how to write effective definitions for technical purposes. This presentation is included with the express permission of the student.

(Technical Definitions, 2015)

Definitions in Technical Writing – Student Presentation


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Technical Description of a Mechanism

(Writing Technical Descriptions , 2015)
Writing Technical Descriptions from Safwan Brightwell

Mechanism descriptions should provide a clear understanding of the object being described, including

  • General appearance and physical properties
  • Overall function/purpose
  • Component parts
  • How the parts interact to create a functioning whole.

The reader should be able to clearly picture, and therefore understand, the nature of the object being described, what it does, and how it works.

In order to achieve this clarity for the reader, the writer must choose significant details and organize information logically. Select details that can be described precisely and measurably, such as

color materials texture, smell, taste
shape component parts finish
size properties patterns, designs
dimensions principles at work interactions

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Depending on the reader’s need, the description may range from a general overview requiring only a few sentences to a multi-chapter manual detailing every aspect of the mechanism’s parts and functions in order to troubleshoot technical problems and complete repairs. For a fun example of the latter, see the Star Trek: The Next Generation: Technical Manual (cover depicted in Figure 7.4.1), which provides detailed descriptions of all equipment and technology used aboard the fictional U.S.S. Enterprise-D.

Cover of manual
Figure 7.4.1 Cover Page of “Star Trek: The Next Generation: Technical Manual” (Sturnback & Okuna, 1991).

Before you begin to draft your description, you must consider your purpose and audience: Why does your audience need this description? What will they use it for? For example, are you describing different types of solar panels for the average consumer to help them choose the one that best fits their needs? Are you giving schematics to technicians and installers?

Once you have your purpose and audience clearly in focus, draft a description that includes the following elements:

  1. Definition: What is it, and what is its main purpose?
  2. Overview: Describe the mechanism’s overall appearance (“big picture”).
  3. Components: Describe the main component parts in labeled sections; consider the order of information carefully here. Create a logical connection between each component described.
  4. Explanation: How do the parts work together to fulfill their function? What key principles govern its functioning? Consider how much detail is necessary here for your intended audience.
  5. Visuals: Include graphics that clearly illustrate the mechanism and/or its parts. Show the device as a whole; consider showing specific details in expanded views, cut-aways, or labeled diagrams. You may even embed or link to videos showing the device in action.
  6. Conclusion: Depending on the purpose, you might review the product’s availability, manufacturing, costs, warnings, etc.)
  7. References: Include sources you have used in your description, or additional sources of information available (if relevant), including specifications, codes, regulations, and manufacturer’s datasheets.

You might consider using a template, like the Technical Description Template below, keeping in mind that while templates can be helpful guides, they do not provide much flexibility and may not work for all situations.

Technical Description Template
Audience and Purpose Who will read this description and why?
Definition and Function What is it? What does it do? What is its purpose?
Overview Describe its overall appearance (shape, size, color, texture, etc.)
Components and Explanations Describe the component parts (chose most relevant features) and explain how they work together. Use descriptive detail related to physical features, like size, weight, colour, texture, materials, composition, etc.
Visuals What kind of illustrative graphics will you use? Where?

  • Diagrams

  • Photographs

  • Cut-away views

  • Exploded views

Conclusion Do you need to offer any further information? History? Warnings? Context? Costs? etc.
References Any sources used, or supplemental sources to suggest.


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Physical characteristics, such as temperature, dimensions, weight, and distance are customarily noted with numerical information that must be written with consistency and accuracy. Learning about technical notations will help you answer questions like: “When do I spell out a number and when do I use a numeral?” “Do I place a space between a number and the value?” “Do I add an ‘s’ for multiples of any value; for example, do I add an ‘s’ to ‘cm’ for centimeters over one?” For a primer on technical notations, see the IEEE article: Using Numbers in Technical Documents (Elliott, 2016). Also check out the APA Style section: Numbers (2019).

Sample Descriptions

Examine the description of the “Up Goer Five” in Figure 7.4.2 (click on image for larger version). Who might the intended audience be?

Blueprint of rocket, labeled using silly-sounding simplistic language such as "fire comes out here"

Figure 7.4.2 A description of the blueprints for NASA’s Saturn Five rocket using only the 1000 most commonly-used English words (Munroe, n.d.).

Compare the description in Figure 7.4.2 to the information given on the NASA website about the Mars Curiosity Rover.

Note the differences in the level of detail, vocabulary, and overall purpose of the descriptions.  If you used the information on the NASA site to fill in the Technical Description Template, you might end up with something like the following chart. Objectives

Template for Description of Mars Curiosity Rover
Definition Curiosity Rover – a NASA robot designed to explore Mars
Function Travels around the Gale Crater on Mars, collecting data to send back to Earth. Its mission is to see if Mars could ever have supported life and if humans could survive there someday
Overview Car-sized, 6-wheel robot, about 7′ tall, with a roughly square chassis that has several appendages connected to it that house sensors of various types
  • Main body protects the computer, electronics and instrument systems

  • “Neck and head” section is like a mast coming out of the centre of the chassis; this houses many of the rover’s cameras

  • Six legs – “rocker bogie” design – wide apart– allows all wheels to remain on uneven terrain

  • Arm – roughly 7’ long, (with “shoulder, elbow and wrist” joints), with a “hand” at the end, extends out of the front of the chassis. This contains many tools for drilling, collecting samples, etc.

  • “Tail” – contains radio-isotopic power source that powers the rover

  • Overall view (front and sides; top view, bottom view)

  • View of arm with labelled components

  • View of head and neck with labeled components

Conclusion/Supplemental Information about lifespan? Travel speed? Energy use?
References NASA website – Mars Curiosity Rover page

You may find that some of these elements are not necessary; again, consider what your target audience already knows. Strike a balance between unnecessarily stating the obvious and incorrectly assuming your readers have knowledge that they lack.

In refining the details of your description and its component parts, consider the following:

  • Organization: Use a logical principle to organize your description
    Figure 7.4.3 Organizing principles used to write technical descriptions (adapted from Isrealov, 2020)
    • Top to bottom (or foundation upward)
    • Left to right (or right to left)
    • Inside to outside (or outside to inside)
    • Most important to least important features
    • Central component to peripherals
    • Material properties, etc.
  • Language:  Use specific, precise, concrete terms – avoid vague or overly general terms
    • Use correct terminology and notations – define terms as necessary for your audience
    • Use analogy to describe an unfamiliar thing in terms of a familiar thing
    • Use objective language – no “ad speak” or subjective terms
    • Use present tense, active verbs to describe how the device appears and what it does
    • Use words that create vivid and specific pictures in the reader’s mind.

EXERCISE 7.4.1 Practice technical description

Choose a common, everyday object (such as one of the objects in Figure 7.4.4) and draft a technical description for an audience unfamiliar with the object. Start by imagining a target audience and purpose, and then try filling in the Technical Description Template with detailed information. Using the information in your template, draft a short description of 1-2 paragraphs, and add properly-captioned visuals.

Figure 7.4.4 Common objects for practice description. Sources: Corkscrew and bicycle images: https://www.flickr.com/photos/dogbomb/527733767 and https://www.flickr.com/photos/8205548@N08/4607907389. CC BY 2.0.

How to Develop Your Descriptions (McMurrey, 1997-2017)

The ability to explain complex, technical matters with ease, grace, and simplicity so that nonspecialist readers understand almost effortlessly is one of the most important skills you can develop as a technical writer. This ability to “translate” difficult-to-read technical discussions is important because so much of technical writing is aimed at nonspecialist audiences. These audiences include important people such as supervisors, executives, investors, financial officers, government officials, and, of course, customers.

Here are some strategies for “translating” technical discussions, that is, specific strategies you can use to make difficult technical discussions easier for nonspecialist readers to understand.

You use your understanding of your audience to decide

  • What information to include in the document
  • What information to exclude from the document
  • How to discuss the information you do include in the document

Translating is particularly important because it means supplying the right kinds of information to make up for the reader’s lack of knowledge or capability. Translating thus enables readers to understand and use your document. Some combination of the techniques discussed in this chapter should help you create a readable, understandable translation:

Defining unfamiliar terms The “in-other-words” technique
Comparing to familiar things Posing rhetorical questions
Elaborating the process Explaining the importance or significance
Providing description Providing illustration
Reviewing theoretical background Providing historical background
Providing examples and applications Providing the human perspective
Shorter sentences and paragraphs Stronger transitions


This list by no means exhausts the possibilities. Other techniques include:

  • Headings. See the section on using headings that break up text and emphasize points and on how to construct headings that guide readers from section to section.
  • Lists. See the section on constructing lists that break up text and emphasize points and on how to construct headings that guide readers from section to section.
Definitions of unfamiliar terms

Defining potentially unfamiliar terms in a report is one of the most important ways to make up for readers’ lack of knowledge in the report subject.

Facial Characteristics of FAS Victims

Taken as a whole, the face of patients with fetal alcohol syndrome (FAS), is very distinctive. Structural deficiencies are thought to be the result of reduced cellular proliferation in the developing stages of the embryo because of the direct action of the alcohol. The face has a drawn-out appearance with characteristics that include short palpebral fissures, epicanthic folds, low nasal bridge, a short upturned nose, indistinct philtrum, small midface, and a thinned upper vermilion.


Figure 2. Facial features characteristic of FAS

Palpebral features are the longitudinal openings between the eyelids. In FAS victims, they tend to be short possibly because the eye size is so small. Most deficiency of the eye is reflected in these shortened palpebral fissures. Epicanthic folds are the vertical folds of skin on either side of the nose, sometimes covering the inner corner of the eye. They are present as a normal characteristic in persons of certain races and also occur as a congenital malformation in patients with FAS. The philtrum is the vertical groove in the middle of the upper lip and below the nose region. It tends to be smooth in patients with fetal alcohol syndrome, and as a consequence the upper lip may lack its usual indentation bow. The upper, red portion of the upper lip is the vermilion; it is often very thin in patients with FAS. The thinned vermilion is a major feature in contributing to the overall drawn-out appearance of the face….

Comparisons to familiar things

Comparing technical concepts to ordinary and familiar things in our daily lives makes them easier to understand. For example, things in the world of electronics and computer—a downright intimidating area for many people—can be compared to channels of water, the five senses of the human body, gates and pathways, or other common things. Notice how comparison (highlighted) is used in these passages:

The helical configuration of the DNA strands is not haphazard. The nitrogen bases on each strand align themselves to form nitrogen base pairs. The pairs are T-A and C-G. Each pair is held together by hydrogen bonds. The pairing of the bases serves to fasten the two helical nucleotide strands together in much the same way as the teeth of a zipper hold the zipper together. The existence of the complementary base pairs explain the constant ratios of T/A and C/G. For every T there must be a complementary A and for every G there must be a complementary C.
David S. Newman, An Invitation to Chemistry (New York: Norton, 1978), pp. 380-381.


All the death and all the misery from a virus so small that 2-1/2 million of them in a line would take up one inch. Flu viruses fall into three types: A, B, and C. Type A, the most variable, causes pandemics as well as regular seasonal outbreaks; type B causes smaller outbreaks and is just now receiving greater attention; type C rarely causes serious health problems.

In appearance, a flu virus somewhat resembles the medieval mace—a ball of iron studded with spikes; moreover, individual part of the flu virus are compared to indivdual parts of the mace. These spikes are two surface proteins called hemagglutinin (HA) and neuraminidase (NA). Inside the virus is a thick tangle of genes. In many other viruses, a number of different genes fit onto one strand of nucleic acid; but each flu gene is a separate segment of ribonucleic acid (RNA)—eight threads in all.

The mace metaphor provides a crude but vivid picture of the influenza virus at work. Hemagglutinin is the substance that in effect bashes into a cell during infection and allows the virus access to the cell interior where it can replicate. Neuramindase permits all the viral offspring to break free of the host cell once replication is complete.Stephen S. Hall, “The Flu,” Science 83, (November 1983), pages 56-57.

Elaborating the process

Explaining in detail the processes involved in the report subject can also help readers. Consider a paragraph like this one, containing only a sketchy reference to the process:

The Video Alert and Control dashboard system, a newly developed system to help drivers avoid accidents, graphically projects an image of hazards in the road.

This brief reference can be converted into a more complete explanation as is illustrated here:

The Video Alert and Control dashboard systems uses a number of components to help drivers avoid accidents. The infrared detector is the key detecting device in that it searches for warm objects in or near the path ahead of the car. The infrared detector senses the upcoming trouble well before the driver by sensing warm-bloodedness and then alerts the driver. The infrared detector also senses the heat of oncoming traffic.

All of these objects are shown graphically on the video screen. To differentiate wildlife from other cars, the x-ray unit is used to check for metal in the object ahead. Thus, if a warm object is detected with metal in it, the computer reads it as a car and shows it on the screen as a yellow dot. On othe other hand, if no metal is detected in the warm object, it is read as an animal and plotted as a red dot….

Providing descriptive detail

Descriptions also help nonspecialist readers by making the report discussion more concrete and down-to-earth:

Jarvik and his colleagues have been working on other designs, such as a portable artificial heart, which they think will be ready for a patient within the next two years.

Electrohydraulic Heart

Jarvik has been developing electric-energy converters and blood pumps during the past year. The electrohydraulic energy converter has only one moving part. The impeller of an axial-flow pump is attached to the rotor of a brushless direct-current motor, with the impeller and the rotor supported by a single hydrodynamic bearing. Reversing the rotation of the pump reverses the direction of the hydraulic flow. The hydraulic fluid (silicone oil of low viscosity) actuates the diaphragm of a blood pump just as compressed air does in the Jarvik-7 heart design. This hydraulic fluid is pumped back and forth between the right and left ventricles.

The energy converter is small and simple and therefore can be implanted without damaging vital structures. It weighs nearly 85 grams and occupies nearly 30 cubic centimeters. The converter requires an external battery and an electronics package, which is connected to the heart by a small cable through the patient’s chest. The batteries weigh 2 to 5 pounds and can be worn on a vest or belt. The battery unit requires new or recharged batteries once or twice a day. The cable through which the power is transmitted from the battery to the heart also carries control signals from the microcomputer controller.


Figure 12. Electrically driven artificial heart system. Source: Jarvik, Robert K. “The Total Artificial Heart,” Scientific American, January 1981, p. 80.Jacqueline R. Mudd, Report on Artificial Methods of Combating Heart Disease, University of Texas at Austin, May 6, 1983.


Providing illustrations

Illustrations—typically, simple diagrams—can help readers understand technical descriptions and explanations of processes. You can see the use of illustration in the FAS example above: epicanthic folds and the philtrum are labels in the diagram.

Providing examples and applications

Equally useful in translating complex or abstract technical discussions are examples or explanations of how a thing can be used. For example, if you are trying to explain a LINUX command, showing how it is used in an example program helps readers greatly. If you are explaining a new design for a solar heating and cooling system, showing its application in a specific home can help also.

Continuous Speech

Continuous speech causes many problems in computerized speech recognition. In fluent speech, many words overlap. For example, when the “t” in “cat” combines the “y” in “your,” the phrase, “You gave the cat your dinner,” sounds like, “You gave the catcher dinner” [8:69].

Some words have built-in pauses that are often longer than word boundaries. For example, the word “vector” has a natural pause between the “c” and the “t.” In an actual experiment, a machine listened to the phrase, “recognize speech,” and printed, “wreck a nice beech” [5:57].

As vocabularies increase, words are more likely to become confused. Some words are subparts of others, such as “plea” and “please,” while some words have similar acoustics, such as “what” and “watt” [23:151].Heidi E. Cootes, Report on Computers that Recognize Speech, University of Texas at Austin, May 6, 1983.

Now here is a passage with a longer, extended example:

…The user “scrolls” the worksheet right and left or up and down to bring different parts of it into view. Each position (that is, each intersection of a column and a row) on a screen corresponds to a record in memory. The user sets up his own matrix by assigning to each record either a label, an item of data or a formula; the corresponding position on the screen displays the assigned the label, the entered datum or the result of applying the formula.

Consider a simple example. A company comptroller might enter the label Cash in the record corresponding to Column B, Row 1 (position B1), Reserves at C1 and Total at D1. He might then enter $300,000 at B2, $500,000 at C2 and the formula +B2+C2 at position D2. The screen will show $800,000 at D2. If the comptroller changes the B2 entry to $200,000, the program will reduce the total displayed at D2 to $700,000. Moreover, what is entered in records B2 and C2 need not be primary data; it can be a function of data held in other recordsHoo-Mi D. Toong and Amar Gupta, “Personal Computers,” Scientific American, (December 1982), pp. 99-100.

Shorter sentences and paragraphs

As simple a technique as it may seem, reducing the length of sentences can make a technical discussion easier to understand. Consider the following pairs of example passages, the second versions of which contain shorter sentences. (The passage still needs other translating techniques, particularly definitions, but the shorter sentences do make it more readable.) Notice too that shorter paragraphs can help in the translation process, not only in the example below but throughout this chapter.

Original version: longer sentences

UV-flourescence was determined on aliquots of the hexane extracts of subsurface water using the Perkin-Elmer MPF-44A dual-scanning flourescence spectrophotometer upon mousse sample NOAA-16, considered the best representative of cargo oil. Every day that samples were processed, a new calibration curve was developed from serial dilutions of the reference mousse (NOAA-16) at an emission wavelength of ca. 360 nm, and other samples were compared to it as the standard. Emission was scanned from 275-500 nm, offset 25 nm from the excitation wavelength, with the major peak occurring at 360 nm for the reference mousse solutions. In each sample, the concentration of flourescent material, a total oil estimate, was calculated from its respective flourescence, using the linear relationship of flourescence vs. concentration of the reference mousse “standard,” with a correction factor applied to account for the reference mousse containing only about 30 percent.

Revised version: shorter sentences

UV-flourescence was determined on aliquots of the hexane extracts of the subsurface water. These measurements were performed using a Perkin-Elmer MPF-44A dual-scanning flourescence spectrophotometer. Mousse sample NOAA-16 was used as the best representative of cargo sample. Other samples were compared to it as the standard.

Every day that samples were processed, a new calibration curve was developed from serial dilutions of the reference mousse (NOAA-16). Tests were run at an emission wavelength of ca. 360 nm. Emission was scanned from 275-500 nm, offset 25 nm from the excitation wavelength. The major peak occurred at 360 nm for the reference mousse solutions.

In each sample, the concentration of flourescent material, a total estimate, was calculated from its respective flourescence. The linear relationship of flourescence vs. concentration of the reference mousse “standard.” A correction factor was applied to account for the reference mousse containing only about 30 percent oil.

Stronger transitions and overviews

Transitions and overviews guide readers through text. In difficult technical material, transitions and overviews are important. (For in-depth discussion, see transitions.)

  • Repetition of key words. As unlikely as it may seem, using the same words for the same ideas is a critical technique for comprehension in technical discussions. In other words, don’t refer to the hard drive as a “fixed-disk drive” one place and as “DASD” (an old IBM term meaning direct access stationary drive) in another. Same goes for verbs: stick with either “boot up” or “system reset,” and don’t vary.
  • Arrangement of key words. Equally important is how you introduce keywords in sentences. If your focus stays on the topic in each sentence of a paragrah, place the keyword at or near the beginning of the second and following sentences. However, if the topic focus shifts from one sentence to the next, use the old-to-new pattern: start the following sentence with the old topic and end the sentence with the new topic. For more detail, see the discussion of topic strings.
  • Transition words and phrases. Examples of transition words and phrases are “for example,” “however,” and so on. When the discussion is particularly difficult and when repetition and arrangement of keywords is not enough, use transition words and phrases. See transitions.
  • Reviews of topics covered and topics to be covered. At certain critical moments within and between paragraph (or groups of paragraphs) occurs a transitional device that either captures what has been discussed in a short phrase, previews what is to be discussed in the following paragraphs, or both. The latter device is also called a topic sentence.

The “in-other-words” technique

Another way of translating technically difficult content is to give the reader two “looks” at the same idea by restating the difficult-to-understand version in simpler terms. The second, simpler explanation is often preceded by a phrase such as “in other words” (IOW). Here are two examples of this IOW technique:

With no electric field present, semiconductor electrons are quite happy to remain bonded in their valence bands. Only when an electric field is applied or the temperature is raised (heat can also increase electron energy) do the valence electrons begin to break their bonds, jump the energy band gap and become conduction electrons.

When a bond is broken, a vacancy or hole is left. The region in which this vacancy exists has a net positive charge. The area where the freed electron exists has a net negative charge. In a semiconductor, both electrons and holes contribute to electrical conduction. If a valence electron from another bond fills the hole without ever gaining sufficient energy to become free, the vacancy appears in a new place. It is as if a positive charge (equal to that on a electron) has moved to a new location. In other words, conduction in semiconductors is the result of two separate and independent particles carrying opposite charges and moving opposite directions under the influence of an applied electric field.David Oakley, Introduction to Semiconductor Theory, University of Texas at Austin, December 12, 1983.


Fatigue is a phenomenon that has plagued engineers for years. It is especially bothersome when metals are involved. Simply stated, fatigue is the slow growth of a crack that ultimately leads to failure after a number of load reversals. A paperclip breaking after repeated bending is an example of fatigue. The process by which fatigue leads to failure can be divided into three stages: initiation, propagation, and failure. The nature of the second stage, propagation, is what enables composites to be immune to failure.

Posing rhetorical questions

In technical writing, you occasionally see questions posed to the readers. Such questions are not there for readers to answer; they are meant to stimulate readers’ curiosity, renew their interest, introduce a new section of the discussion, or allow for a pause:

When an animal runs, its legs swing back and forth through large angles to provide balance and forward drive. We have found that such swinging motions of the leg do not have to be explicitly programmed for a machine but are a natural outcome of the interactions between the controllers for balance and attitude. Suppose the vehicle is traveling at a constant horizontal rate and is landing with its body upright. What must the attitude controller do during the stance to maintain the upright attitude? It must make sure that no torques are generated at the hip. Since the foot is fixed on the ground during stance, the leg must sweep back through an angle in order to guarantee that the torque on the hip will be zero while the body moves forward.

On the other hand, what must the balance servo do during flight to maintain balance? Since the foot must spend about as much time in front of the vehicle’s center of gravity as behind it, the rate of travel and the duration of stance dictate a forward foot position for landing that will place the foot in a suitable spot for the next stance period. Thus during each flight the leg must swing forward under the direction of the balance servo, and during each stance it must sweep backward under the control of the attitude servo; the forward and back sweeping motions required for running are obtained automatically from the interplay of the servo-control loops for balance and attitude.

Two-Dimensional Hopping Machine

Marc H. Raibert and Ivan E. Sutherland, “Machines that Walk,” Scientific American, (January 1983), p. 50.

Explaining the importance

Some translating techniques work because they motivate readers. Sometimes readers need to be talked into concentrating on difficult technical discussion: one way is to explain to them or to remind them of the importance of what is being discussed. In this example, the last paragraph emphasizes the importance):

It was Linus Pauling and his coworkers who discovered that sickle cell anemia was a molecular disease. This disease affects a very high percentage of black Africans, as high as 40 percent in some regions. About 9 percent of black Americans are heterozygous for the gene that causes the disease. People who are heterozygous for sickle cell anemia contain one normal gene and one sickle cell gene. Since neither gene in this case is dominant, half the hemoglobin molecules will be normal and half sickled. The characteristic feature of this disease is a sickling of the normally round, or platelike, red blood cells under conditions of slight oxygen deprivation. The sickled red blood cells clog small blood vessels and capillaries. The body’s response is to send out white blood cells to destroy the sickled red blood cells, thus causing a shortage of red blood cells, or anemia.

The sickle cell gene originated from a mistake in information. A DNA molecule somehow misplaced a base, which in turn caused an RNA molecule to direct the cell to make hemoglobin with just one different amino acid unit among the nearly 600 normally constituting a hemoglobin molecule. So finely tuned is the human organism that this tiny difference is enough to cause death.

Since the disease is nearly always fatal before puberty, how can a gene for a fatal childhood disease get so widespread in a population? The answer to this question gives some fascinating insight into the mechanism and purposes of evolution, or natural selection. The distribution of sickle cell anemia very closely parallels the distribution of a particularly deadly malaria-causing protozoan by the name of Plasmodium falciparum, and it turns out that there is a close connection between sickle cell anemia and malaria. Those people who are heterozygous for the sickle cell gene are relatively immune to malaria and, except under reasonably severe oxygen deprivation such as that found at high altitudes, they experience no noticeable effects due to the sickle cell gene they carry. Half the hemoglobin molecules in the red cells of heterozygous people are normal and half are sickled. Thus, under ordinary circumstances the normal hemoglobin carries on the usual respiratory functions of blood cells and there is little discomfort. On the other hand, the sickled hemoglobin molecules precipitate, in effect, when the malaria-causing protozoan enters the blood. The precipitated hemoglobin seems to crush the malaria protozoan, thus keeping the malaria from being fatal.

The significance of all this should be pondered. Nature is willing to sacrifice approximately half the children in the malaria-infested regions of Africa so that the species can survive. The reason half the children die is that, on the average, approximately one-quarter of the children will be homozygous for abnormal hemoglobin and will die of sickle cell anemia, while one-quarter will be homozygous for normal hemoglobin and will likely die of malaria. The half of the population that is heterozygous will survive to reproduce. This means that the species, not the individual, is the ultimate unit of Darwinian evolution.David S. Newman, An Invitation to Chemistry, pages 387-388.

Providing historical background

Discussion of the historical background of a technical subject helps readers because it gives them less technical, more general, and sometimes more familiar information. It gives them a base of understanding from which to launch into the more difficult sections of the discussion:

Now that alcohol is being used in more and more social settings, it extremely important to recognize its teratogenic effects. Teratogenic, or malforming, agents produce an abnormal presence or absence of a substance that is required in physical development. Although Sullivan first reported on the effects of maternal drinking during pregnancy in 1899, the serious implications of his findings were virtually ignored for the next 50 years. It was not until the dramatic identification of a pattern of malformations, termed the fetal alcohol syndrome (FAS) by Jones et al in 1973, that the scientific community acknowledged the potential dangers of heavy maternal alcohol use. Since then, there has been increasing recognition that alcohol may be the most common drug in causing problems of malformations in humans.

Each morning in the soft, coral flush of daybreak, a laser dawns on Mars. Forty miles above frigid deserts of red stone and dust, it flares in an atmosphere of carbon dioxide. Infrared sunlight kindles in this gas a self-intensifying radiance that continuously generates as much energy as a thousand nuclear reactors. Our eyes are blind to it, but from sunrise to sunset Mars bathes in dazzling lasershine.
The red planet may have lased in the sun for eons before astronomers identified its sky-high natural laser in 1980. The wonder is that its existence was unknown for so long. In 1898, in The War of the Worlds, H.G. Wells scourged earth with Martian invaders and a laserlike death ray. Pitiless, this “ghost of a beam of light” blasted brick, fired trees, and pierced iron as if it were paper.

In 1917 Albert Einstein speculated that under certain conditions atoms or molecules could absorb light or other radiation and then be stimulated to shed their borrowed energy. In the 1950s Soviet and American physicists independently theorized how this borrowed energy could be multiplied and repaid with prodigious interest. In 1960 Theodore H. Maiman invested the glare of a flash lamp in a rod of synthetic ruby; from that first laser on earth he extorted a burst of crimson light so brilliant it outshone the sun.Allen A. Boraiko, “The Laser: ‘A Splendid Light,'” National Geographic, (March 1984), p.335.

Reviewing theoretical background

To understand some phenomena, technologies, or their applications, readers must first understand the principle or theory behind them. Theoretical discussions need not be over the heads of nonspecialist readers. Discussion of theory is often little more than explanation of the root causes and effects at work in a phenomenon or mechanism. In this example, the writer establishes the theory and then can go on to discuss the findings that have come about through the use of NMR on living tissue.

To the extent that objections persist about the validity of modern biochemistry, they continue to be about reducing the processes of life to sequences of chemical reactions. “The reactions may take place in the test tube,” one hears, “but do they really happen that way inside the living cell? And what happens in multicellular organisms?”

One technique is beginning to answer these questions by detecting chemical reactions as they occur inside cells, tissues and organisms including those of human beings. The technique is nuclear-magnetic-resonance (NMR) spectroscopy. It relies on the fact that atomic nuclei with an odd number of nucleons (protons and neutrons) have an intrinsic magnetism that makes each such nucleus a magnetic dipole: in essence a bar magnet. Such nuclei include the proton (H-1), which is the nucleus of 99.98 percent of all hydrogen atoms occurring in nature, the carbon-13 nucleus (C-13), which is the nucleus of 1.1 percent of all carbon atoms, and the phosphorus-31 nucleus (P-31), which is the nucleus of all phosphorus atoms.

Combining the translating techniques

This last section concludes the techniques for translating difficult technical prose to be presented here. However, take a look at writing in fields you know about, and look for other kinds of translating techniques used there. Now, here are several extended passages of technical writing that combine several of these strategies.

Fine-tuning the spectrum

To know lasers, one must first know the electromagnetic spectrum, which ranges from long radio waves to short, powerful gamma rays.

The narrow band of the spectrum we know as visible, or white, light is made up of red, orange, yellow, green, blue, and violet light. These frequencies, as well as all radiation waves, are jumbled or diffused, much as noise is a collection of overlapping, interfering sounds. Laser light is organized and concentrated, like a single, clear musical note.

In lasers, nature’s disorder is given coherence, and photons—the basic units of all radiation—are sent out in regular ranks of one frequency. Because the waves coincide, the photons enhance one another, increasing their power to pass on energy and infomation.

The first devices to emit concentrated radiation operated in the low-energy microwave frequencies. Today, laser technology is extending beyond ultraviolet toward the high-energy realms of x-rays. Each wavelength boasts its own capacities as a tool for man.

A laser’s beam can be modulated into an infinite number of wavelengths using flourescent dyes like those produced at Exciton Chemical Company in Ohio. At Hughes Research Laboratories in California, a blue-green laser reflected at an acute angle aneals silicon microchips, while a low-energy red laser monitors the process.

Harnessing light

As a bow stores energy and releases it to drive an arrow so lasers store energy in atoms and molecules, concentrate it, and release it in powerful waves.

When an atom expands the orbits of its electrons, they instantly snap back, shedding energy in the form of a photon. When a molecule vibrates or changes its geometry, it also snaps back to emit a photon.

In most lasers a medium of crystal, gas, or liquid is energized by high-intensity light, an electric discharge, or even nuclear radiation. When a photon reaches an atom, the energy exchange stimulates the emission of another photon in the same wavelength and direction, and so on, until a cascade of growing energy sweeps through the medium.

The photons travel the length of the laser and bounce off mirrors—one a full mirror, one partially silvered—at either end. Photons, reflected back and forth, finally gain so much energy that they exit the partially silvered end, emerging as powerful beam.

Out of the darkness: laser eye surgery

Sight-saving shafts of light able to enter the eye without injuring it, lasers are revolutionizing eye surgery.

Using techniques of New York opthalmologist Frances L’Esperance, eye surgeons employ four levels of laser energy. Exposure time ranges from 30 minutes for low-energy photoradiation to several billionths of a second for photodisruption.

With microscopic focus, beams weld breaks in the retina or seal leaking blood vessels by photocoagulation. A painless 20-minute operation call an irridectomy relieves this excess fluid buildup of glaucoma.

When an artificial lens is placed behind the iris, the supportive membrane often grows milky. A laser beam is pinpointed on the taut tissue in a series of minute explosions. This photodisruption causes the tissue to unzip and part like a curtain. Bloodless scalpels, lasers can make extremely delicate incisions, cauterize blood vessels, and leave tissue unaffected only a few cell widths away.

Beams that heal

Surgical trauma, the jarring aftermath of the surgeon’s knife, may one day be consigned to the annals of primitive medicine—thanks to a procedure called “least invasive surgery” by its growing number of practitioners. Using an endoscope, surgeons can view the interior of the body and operate with the least amount of damage.

An end view of the flexible tube … shows a large optical fiber to light the way. Smaller openings facilitate fluid suction and gas suction. A forceps, controlled by a cable near the microscope viewing lens, extracts tissue for analysis. A laser, controlled by dials to the left of the eyepiece, streams from another tube, ready to perform wherever the doctor directs it. Twisting and probing wit the end of the scope, he can identify and coagulate a bleeding ulcer in the stomach or blast tumors in the esophagus. The beam is fed through the scope by an optical fiber from a laser machine … that might cost the hospital from $20,000 to $150,000.Allen A. Boraiko, “Lasers: A Splendid Light,” National Geographic, (March 1984), pp. 338-346.




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