Guidelines
for Writing Good Descriptions
Organization
- OVERVIEW. Begin with a brief overview that reveals the object's
(a) overall framework, arrangement, or shape, and
(b) purpose or function.
- PARTS. Divide the object into parts and describe each part
(a) in enough detail to use, make, or draw it, and
(b) in a way that reveals its role, its relation to other parts.
- ORDER. Organize the part descriptions to help your reader:
(a) spatial order (top to bottom, outside to inside), or
(b) priority order (most to least important), or
(c) chronological order (order of [dis]assembly).
Content
- SPECIFICS.
- Include relevant specific features (such as size, shape, color,
material, technical names).
- Omit irrelevant background, confusing details, and needless words.
- COMPARISON. Compare features or parts with other things already
familiar.
- CONTRAST. Contrast properties with different ones to reveal
their significance.
Signals for Your Reader
- FORMAT. Clarify your text with:
- Heads. Identify topics with clear, nested section headings.
- Lists. Itemize related features with indenting and marks.
- Figures. Integrate figures and text with labels and references.
- VERBAL CUES. Guide your reader's expectations with:
- Parallelism. Use parallel words and phrases for parallel ideas.
- Proleptics. Use verbal links (also, but, however, etc.)
to signal how your description fits together.
Exercise 1: Paper Clip
Description Case 1: Paper Clip
Description
A "Gem-style" paper clip is a FEATURE:
length of stiff steel wire bent
into three flat, nested loops WHY:
(Fig. 1) to hold sheets of paper
together when they are inserted
between the loops.
The wire is a 1-mm-diameter
steel cylinder that is 10 cm FEATURE:
long. It is bendable in the WHY:
fingers but stiff.
The first loop (a) is a smooth,
U-shaped turn to the right
that starts 2 cm from the
outermost end of the wire.
The second loop (b) is a FEATURE:
U-shaped turn to the left WHY:
that starts 3 cm farther
along the wire and has a FEATURE:
diameter just small enough WHY:
to fit snugly within the
first loop.
The third loop (c) is another
U-shaped turn to the right
that starts 2 cm beyond (b)
and has a diameter just small
enough to fit snugly within
the second loop (as well as
the first).
The wire in each inner loop FEATURE:
touches and runs parallel to WHY:
the outer loop that wraps
around it. All three loops
lie in the same plane, and
pushing them out of that
plane just enough to slide
several sheets of paper
between them makes the paper FEATURE:
clip act like a spring and WHY:
squeeze the sheets together.
Exercise 2: Nail Clippers
Description Case 2: Nail Clippers
Description Analysis
Nail clippers combine two steel FEATURE:
levers to make a strong, stable WHY:
tool that clips off the end of a
finger nail with little applied
force and much control.
Clippers consist of three steel FEATURE:
strips about 1 cm wide, 5 cm WHY:
long, and 1 mm thick.
A steel post (3 mm in diameter
and 1 cm long) connects all
these strips (Fig. 1).
The bottom strip is riveted
to the post at right angles;
the other two strips fit over
the post through a circular
hole in each that lets them
move freely along its length.
FEATURE:
The top strip forms the handle WHY:
of the clippers. It bends
upward at a 45-degree angle
about one fifth of the way
from the end that passes over
the post, against which the
the handle's short end pivots.
The bottom strip is straight, FEATURE:
with a short 90-degree bend and WHY:
beveled cutting edge on the end
nearest the post.
The middle strip gently
bends upward about 10 degrees
near the end away from the post.
It is welded at that end to the
bottom strip (below it).
At the other end, which is free
to move, it has a short vertical
section (bent toward the lower
strip), also with a beveled
cutting edge.
The handle (top strip) forms a FEATURE:
second-class lever, with its WHY:
fulcrum at the post (F in
Fig. 2). Gentle force moves
the long end through a long
distance, applying high force
(at the bend) to the middle
of the strip below it.
The middle strip forms a FEATURE(S):
third-class lever, with its
fulcrum (F) at the welded WHY:
end. High force applied to
its middle by the handle bend
(above it) moves the cutting
edge gently through enough
distance to meet the facing
edge below it, carefully
cutting any finger nail
inserted between the beveled
edges.
Exercise 3: Compact Disk (Outline)
Description Case 3: Compact Disk
Overview
General Shape
Size
Capacity
The Layers
The Groove
One-track Design
Moving the Groove
The Pits
The Optical Read-Out System
Exercise 3: Compact Disk (Scaffolded)
Description Case 3: Compact Disk
Description Analysis
Overview
A compact disk (CD), like a FEATURE:
phonograph record, stores WHY:
information physically for
electronic replay, and, like a
phonograph record, it stores the
information along a single spiral
groove on the side of a plastic
circle.
In contrast to a phonograph FEATURE:
record, however, a CD WHY:
* stores the information in
digital (on/off) rather than
analog (variable shape) form,
and
* is read by reflected (laser)
light, rather than by vibrating
a needle that travels along the
groove.
General Shape
Size
A compact disk is a circle of FEATURE:
clear plastic (polycarbonate) WHY:
about 12 cm in diameter and
1 mm thick, with a 1.5-cm
diameter hole in the center.
CDs are stamped from a mold that
leaves a spiral track lined with
pits (little dents) on the CD's
bottom side (details below),
while the top side is smooth.
Capacity
The surface area of a CD is just FEATURE:
less than twice (1.77 times) as WHY:
large as the surface area of a
3.5-inch magnetic "floppy" disk.
But because the pits store
information much more densely
than the iron oxide particles on
a floppy disk, a CD holds at
least 350 times more data (at
least 500 Mbyte on a CD, only
1.44 Mbyte a magnetic disk).
The Layers FEATURE:
WHY:
CDs consist of three layers (see
Fig. 1):
(a) The bottom layer is the FEATURE:
stamped, grooved plastic. WHY:
(b) Above that lies a very thin
film of aluminum (or chrome-
aluminum alloy). This
metallic film reflects any FEATURE:
light entering the disk from WHY:
below, except where the pits
fall in the plastic layer.
(c) Above the reflective metal
layer is a coat of acrylic
lacquer that protects the
metal from scratches and
oxidation. It also allows
printing descriptive labels
safely on the top side of
the disk.
The Groove
One-track Design
The groove on the bottom surface
of a CD is a single channel that
spirals outward from the center
to the edge. This track is FEATURE:
thinner than a human hair and WHY:
several kilometers long.
(The spiral differs from the FEATURE:
many concentric rings of iron WHY:
oxide (see Fig. 2) that store
information on a magnetic disk.)
Moving the Groove
The spindle through a CD's center
hole (see Fig. 3, d) connects
the disk to a variable-speed FEATURE:
motor (unlike the constant-speed WHY:
motor on a phonograph turntable).
The disk turns (clockwise) about
500 revolutions/minute (e) when FEATURE:
the reading laser beam is at the WHY:
center, but only about 200
revolutions/minute when the beam
reaches the outer edge.
This causes the track to pass
over the read-out system (f),
which gradually moves from the
center to the edge, at a
constant linear speed, to help
reliably detect the pits.
The Pits
The moving spiral track is lined FEATURE:
with pits (dents) and flat spots WHY:
("lands"). These vary in size
and placement in a sequence that
represents the information stored.
The pit sequence can digitally FEATURE:
encode text, images, computer WHY:
programs, or the left- and right-
hand audio signals of a stereo
sound recording.
Additional pits
* give location and timing
information (for player
display), and
* control the motor speed
so that the reading rate
remains constant.
The Optical Read-Out System
Two lenses and a semi-transparent FEATURE:
(partially silvered) mirror WHY:
(see Fig. 4) direct the laser
beam from below at the track on
the spinning CD.
If the laser beam strikes a pit FEATURE:
on the track (g), it is not WHY:
reflected. The light-sensitive
photodiode (detector) below the
mirror sees no beam and produces
no signal.
If the laser beam strikes a land
between pits on the track (h),
it reflects back straight through
the mirror to the photodiode
below. This detector then
produces an electric signal.
For compatibility with other
electronic equipment, a special
reversing circuit (a "not gate")
then turns these pit
interruptions into ON signals
(binary 1s) and turns the land
reflections into OFF signals
(binary 0s).
Exercise 3: Compact Disk (Segmented)
Description Case 3: Compact Disk
---
A compact disk (CD), like a
phonograph record, stores
information physically for
electronic replay, and, like a
phonograph record, it stores the
information along a single spiral
groove on the side of a plastic
circle.
---
In contrast to a phonograph
record, however, a CD
* stores the information in
digital (on/off) rather than
analog (variable shape) form,
and
* is read by reflected (laser)
light, rather than by vibrating
a needle that travels along the
groove.
---
A compact disk is a circle of
clear plastic (polycarbonate)
about 12 cm in diameter and
1 mm thick, with a 1.5-cm
diameter hole in the center.
CDs are stamped from a mold that
leaves a spiral track lined with
pits (little dents) on the CD's
bottom side (details below),
while the top side is smooth.
---
The surface area of a CD is just
less than twice (1.77 times) as
large as the surface area of a
3.5-inch magnetic "floppy" disk.
But because the pits store
information much more densely
than the iron oxide particles on
a floppy disk, a CD holds at
least 350 times more data (at
least 500 Mbyte on a CD, only
1.44 Mbyte a magnetic disk).
---
CDs consist of three layers (see
Fig. 1):
(a) The bottom layer is the
stamped, grooved plastic.
(b) Above that lies a very thin
film of aluminum (or chrome-
aluminum alloy). This
metallic film reflects any
light entering the disk from
below, except where the pits
fall in the plastic layer.
---
(c) Above the reflective metal
layer is a coat of acrylic
lacquer that protects the
metal from scratches and
oxidation. It also allows
printing descriptive labels
safely on the top side of
the disk.
---
The groove on the bottom surface
of a CD is a single channel that
spirals outward from the center
to the edge. This track is
thinner than a human hair and
several kilometers long.
(The spiral differs from the
many concentric rings of iron
oxide (see Fig. 2) that store
information on a magnetic disk.)
---
The spindle through a CD's center
hole (see Fig. 3, d) connects
the disk to a variable-speed
motor (unlike the constant-speed
motor on a phonograph turntable).
---
The disk turns (clockwise) about
500 revolutions/minute (e) when
the reading laser beam is at the
center, but only about 200
revolutions/minute when the beam
reaches the outer edge.
This causes the track to pass
over the read-out system (f),
which gradually moves from the
center to the edge, at a
constant linear speed, to help
reliably detect the pits.
---
The moving spiral track is lined
with pits (dents) and flat spots
("lands"). These vary in size
and placement in a sequence that
represents the information stored.
The pit sequence can digitally
encode text, images, computer
programs, or the left- and right-
hand audio signals of a stereo
sound recording.
---
Additional pits
* give location and timing
information (for player
display), and
* control the motor speed
so that the reading rate
remains constant.
---
Two lenses and a semi-transparent
(partially silvered) mirror
(see Fig. 4) direct the laser
beam from below at the track on
the spinning CD.
---
If the laser beam strikes a pit
on the track (g), it is not
reflected. The light-sensitive
photodiode (detector) below the
mirror sees no beam and produces
no signal.
If the laser beam strikes a land
between pits on the track (h),
it reflects back straight through
the mirror to the photodiode
below. This detector then
produces an electric signal.
---
For compatibility with other
electronic equipment, a special
reversing circuit (a "not gate")
then turns these pit
interruptions into ON signals
(binary 1s) and turns the land
reflections into OFF signals
(binary 0s).
---
Exercise 4: Post-it Note (Outline)
Description Case 4: Post-it Note
Overview
The Paper
The Adhesive
Exercise 4: Post-it Note (Scaffolded)
Description Case 4: Post-it Note
Description Analysis
Overview
A Post-it note is an easy way FEATURE:
to temporarily annotate a WHY:
document by applying a small
square of colorful, durable
paper using a strip of
repositionable adhesive on the
back of the note.
The Paper FEATURE:
WHY:
The most common Post-it notes
are 1.5-by-2-inch rectangles
of nonwhite (usually yellow)
paper available in pads of 100.
However, 55 larger sizes and
shapes (up to poster size) are
also available.
Post-it paper is well suited to FEATURE:
making reliable notes because it: WHY:
(1) does not tear or fray easily,
even after repeated uses,
(2) is highly opaque, resisting FEATURE:
bleed-through from ink or WHY:
felt-tip pens, and
(3) comes in 29 colors that
visually contrast with the
document pages to which the
notes are applied.
The Adhesive
The adhesive that holds the note
to its target page lies in a
half-inch strip along the top
edge of the back of each Post-it.
Post-it adhesive consists of FEATURE:
tiny sticky spheres that do not WHY:
easily dissolve or melt, and
that have about the same diameter
as the paper fibers they touch.
This adhesive therefore combines
several unusual properties.
First, the adhesive is clear and FEATURE:
thinner than standard plastic WHY:
mounting tape.
Second, unlike an adhesive FEATURE:
bandage, it leaves no residue on WHY:
the page to which the Post-it is
applied.
Third, the adhesive is long-
lasting while undisturbed;
Post-it notes will cling for
months (at room temperature)
before falling off their applied
surfaces.
And fourth, the adhesive is also
reusable.
A clean Post-it may be removed
and reapplied in the same or a
different location dozens of
times before the adhesive strip
fails to hold the note to its FEATURE:
target (unlike most tape). WHY:
Art Fry of 3M Corp. first FEATURE:
developed the Post-it note in WHY:
1980.
Exercise 4: Post-it Note (Segmented)
Description Case 4: Post-it Note
---
A Post-it note is an easy way
to temporarily annotate a
document by applying a small
square of colorful, durable
paper using a strip of
repositionable adhesive on the
back of the note.
---
The most common Post-it notes
are 1.5-by-2-inch rectangles
of nonwhite (usually yellow)
paper available in pads of 100.
---
However, 55 larger sizes and
shapes (up to poster size) are
also available.
---
Post-it paper is well suited to
making reliable notes because it:
---
(1) does not tear or fray easily,
even after repeated uses,
---
(2) is highly opaque, resisting
bleed-through from ink or
felt-tip pens, and
---
(3) comes in 29 colors that
visually contrast with the
document pages to which the
notes are applied.
---
The adhesive that holds the note
to its target page lies in a
half-inch strip along the top
edge of the back of each Post-it.
---
Post-it adhesive consists of
tiny sticky spheres that do not
easily dissolve or melt, and
that have about the same diameter
as the paper fibers they touch.
---
This adhesive therefore combines
several unusual properties.
---
First, the adhesive is clear and
thinner than standard plastic
mounting tape.
---
Second, unlike an adhesive
bandage, it leaves no residue on
the page to which the Post-it is
applied.
---
Third, the adhesive is long-
lasting while undisturbed;
Post-it notes will cling for
months (at room temperature)
before falling off their applied
surfaces.
---
And fourth, the adhesive is also
reusable.
---
A clean Post-it may be removed
and reapplied in the same or a
different location dozens of
times before the adhesive strip
fails to hold the note to its
target (unlike most tape).
---
Art Fry of 3M Corp. first
developed the Post-it note in
1980.
---
Exercise 5: Fluorescent Lamp (Outline)
Description Case 5: Fluorescent Lamp
Overview
Structure
Size
Contents
Labels
Operation
Gas Discharge
STARTING.
EMITTING.
VISIBILITY.
Wavelength Conversion
Efficiency
HEAT/LIGHT RATIO.
BULB LONGEVITY.
Exercise 5: Fluorescent Lamp (Segmented)
Description Case 5: Fluorescent Lamp
---
A fluorescent lamp is a long
straight glass tube that glows
when a current passing through
low-pressure gas within the tube
causes a coating on the glass to
emit white light.
---
Fluorescent lamps were first
introduced commercially in 1938.
---
A standard fluorescent lamp is a
cylindrical glass tube 1.5 inches
in diameter and 48 inches long
(other sizes are available).
---
A 2-pin metal base or cap seals
each end of the tube (see Fig. 1).
---
Inside each end cap, attached to
the pins, is a filament or
electrode, a thin thread of wire
from which electrons boil when it
is heated by an electric current.
---
Sealed within the tube by the
caps is a drop of mercury and a
very low-pressure inert gas
(usually argon).
---
A light-emitting chemical (see
the Operation section) called a
phosphor coats the entire inside
surface of the glass.
---
Fluorescent lamps carry
standardized labels outside that
identify their internal physical
and electrical properties.
---
For example, a lamp with the
black characters
F40-T12
stenciled on one end is a
fluorescent (F) tube (T) that
uses 40 watts of power and has
a diameter of 12 eighths of an
inch (12/8 = 3/2 = 1.5 inch).
---
STARTING. When the lamp is off,
the mixture of mercury and gas
inside does not conduct
electricity.
---
So every fluorescent lamp is
attached to a starting device
called a ballast, which combines
* a "transformer" to produce an
initial, high-voltage burst,
and
* an "inductor" to limit
current flow while the lamp
is on.
---
EMITTING. When power is first
applied, a 250- to 400-volt burst
of electricity vaporizes the
mercury (see Fig. 1, left).
---
Electrons in the mercury atoms
absorb energy and jump to
"higher," more energetic orbits
(Fig. 1, middle).
---
They then fall back to less
energetic orbits (Fig.1, right).
---
This repeating process, called
gas discharge, continuously
emits the absorbed energy as
light.
---
Once started, only about 175 volts
are needed to maintain this
discharge in a 40-watt lamp.
---
VISIBILITY. When an applied
voltage causes discharge in some
low-pressure, inert gases, they
emit visible light.
---
Ionized neon gas emits red
light, for example, seen
directly in a glowing neon bulb.
---
But in a fluorescent lamp, the
discharge comes almost entirely
from the mercury vapor, even
though it is only 1 percent of
the enclosed gas.
---
And almost all of the mercury
discharge is ultraviolet (UV)
light, whose wavelength is too
short for human eyes to see.
---
The phosphor that coats the
inside of the lamp tube converts
the UV mercury discharge into
useful light that people can see.
---
The phosphor absorbs the
invisible, short-wave UV
emissions from the excited
mercury atoms (Fig. 1, right).
---
It then emits other light with
a longer wavelength, almost all
of which is visible.
---
The chemical composition of the
phosphor lining the tube controls
the color of the visible light
emitted, which may be
* "cool white" (partly blue), or
* "warm white" (partly pink), or
* other visible colors.
---
HEAT/LIGHT RATIO. All lamps
convert current into visible
light and heat.
---
Fluorescent lamps are about 2 to
4 times more efficient than
incandescent (glowing filament)
lamps.
---
For the same power, they produce
2 to 4 times more light and less
heat.
---
BULB LONGEVITY. Fluorescent
lamps also have longer lifetimes.
A typical incandescent bulb lasts
1000 hours before the filament
fails.
---
But a typical fluorescent lamp
lasts 10,000 to 20,000 hours,
depending on how often it is
started.
---
Exercise 9: Revising Wisely (Bone Fracture)
How Old Bones Fracture [Lines Numbered for Reference]
1
2 Old bones fracture more easily than
3 young ones, even under small amounts
4 of stress.
5 This is partly because old bones lose
6 density, but partly because they also
7 change their structure.
8
9 Many human bones contain two different
10 internal structures.
11 Some central tissue is quite solid
12 (cortical bone).
13 Other tissue, however, contains a
14 cellular foam or open-celled lattice
15 of collagen (trabecular bone).
16 Trabecular bone occurs in
17 * individual parts of the spine
18 (vertebrae),
19 * the femur (the long bone that joins
20 the knee to the hip),
21 * the tibia (the larger of the two
22 bones connecting the knee to the
23 ankle),
24 * the humerus (the long bone linking
25 the shoulder to the elbow), and
26 * the iliac crest (the widest part of
27 the hip bone).
28
29 Trabeculae are like sponges.
30 They absorb loads from the joints just as
31 Styrofoam absorbs impacts in packages.
32
33 All bone renews itself regularly.
34 The human body continuously removes
35 old bone (resorption) and replaces it
36 with new bone (formation).
37 The average bone turnover rate is
38 about 6 years.
39
40 In menopausal women, however, this
41 turnover process becomes unbalanced.
42 More bone is resorbed that is formed,
43 so bone density decreases (osteoporosis).
44 Medical treatments for osteoporosis
45 usually focus on stopping resorption
46 of old bone while still allowing new
47 bone to fill in.
48 Reducing turnover in this way leaves
49 bone more massive yet also more brittle.
50
51 Three-dimensional computed-tomography
52 images reveal a structural change in
53 human trabecular bone with age.
54 In young bone, the trabeculae form a web
55 of short, stubby struts.
56 In older bone, on the other hand, these
57 trabeculae are mostly long, slender
58 columns.
59
60 Fracture (failure) in younger bones
61 usually happens when stress overcomes
62 the strength of the bone tissue.
63 Fracture in older bones happens at
64 much lower stress levels.
65 The long, thin trabeculae in old bones
66 are unstable.
67 Hence, when compressed, they buckle at
68 far lower stresses than bone tissue
69 normally withstands.
70
71 Bone resorption forms small pits on the
72 surface of the trabeculae.
73 As bone turnover increases, the number
74 of pits increases too.
75
76 An empty aluminum can requires much
77 stress to crush it axially.
78 But with a small dent in its side wall
79 the can buckles easily.
80 In the same way, trabecular pits are
81 themselves enough to destabilize
82 bone lattice.
83 In old human bone, 60% of trabeculae
84 are vulnerable to buckling just because
85 of their pits.