A local collection of pus within a body cavity having a range of symptoms.
Abscesses are serious and must be treated promptly.
An abrasive is a harder material used to
smooth a rough surface, rough a smooth surface, or change the shape or size of a softer
material by rubbing or grinding. The area worn away
between the rubbed materials is called an abrasion. Abrasives
are used in 3 ways: (1) Placing the sharpened object against the abrasive, e.g., sharpening a
knife or axe against a grinding wheel made of stone. (2) Coat a substance with an
Examples are sandpaper, emery, natural or synthetic diamond dust, CBN (cubic
boron nitride), silicon carbide ("carborundum" = SiC).
For applications against wood, the abrasive is attached to a substrate (e.g., sandpaper) with a
glue and rubbed by hand or a motor-driven sander. For metal applications, it is made into a
wheel that is attached to an upright stand and driven by an electric
motor. The stand type is always found in
machine shops because of the frequent need to deburr cut
metal. (3) Placing the abrasive in a high-pressure air or water stream and pushing them
against the surface to be abraded, e.g., sandblasting or gritblasting. There are hundreds
of abrasive compounds in use. Cleaning compounds contain abrasives using silica,
pumice, or aluminum oxide. Toothpaste contains powdered chalk as its abrasive.
Abrasives contribute to health by preventing lacerating burrs,
called abrasions, and to precise tolerances by removing small amounts
of material beyond the capability of a cutting tool (e.g., wood used for furniture).
Before the invention of carborundum (see below), all abrasives were natural minerals.
Examples are sand, garnet, emery, corundum, and pumice.
Sandpaper was patented in the United States on June 14, 1834 by Isaac Fischer, Jr., of
In 1891, Edward Acheson produced crystals of silicon carbide (SiC), the first synthetic
abrasive, which he patented and sold under the trade name, Carborundum. The SiC
crystals can be made at any degree of fineness and can be bonded together into a solid block
or coated onto metal discs or belts.
- Absolute Zero
The temperature at which there is no molecular motion and therefore no
It is measured as 0°K = -273.15°C = -459.67°F.
The rate of change of velocity with respect to time. a = dv / dt, where
dv is an increase or decrease in velocity and dt is its corresponding increase in time.
Since the time change is always positive, an increase in velocity results in a positive acceleration,
while a decrease in velocity results in a negative acceleration; that is, a
- Accuracy, Precision & Tolerances
Accuracy is the closeness of a dimension to some accepted standard. The
dimension can be length, area, volume, time, energy, power, etc. How accurate is the
inch measure on
a ruler to the international standard? The area of a built house to its plan? The
volume of a bottle to its intended volume? The time at noon of a watch to the
international time standard at noon? The amount of power of a motor to its
designed power rating? Accuracy is a relative measure to some standard of acceptability.
Tolerances are the limits of acceptable accuracy. A ruler
inch can be within +/-
.00001 inch for a metal shop ruler, +/- .0001 inch for a school ruler. The area of a specific
house can be + 20.0/- 0.0 square feet from plan. The volume of a specific bottle may
+ .100/-.005 quarts from its intended volume. The time at noon of a watch over 1
24-hour period may be +/- .10 second. The amount of power actually generated by a
motor can be +10/-1 hp (horsepower) from its rated power. Tolerances are determined by
the amount of accuracy required for a part to function properly alone or in conjunction with other
parts in an assembly. For examples, if a bullet is too large, then it will not enter the rifle
barrel to fire. If a gear is too small in a transmission, it will not mesh properly with other
gears and the transmission will fail.
Precision is the measure of the repeatability or reliability of
accuracy. If 999 of a batch of 1000 metal shop rulers are within the acceptable
tolerances, then they are more reliable than another batch that has 997 rulers out of 1000 within
the acceptable tolerances. If 999 triggers out of 1000 in a pistol function properly, then the
precision is 99.9%. For interchangeable parts
manufacture, precision must be
high enough so that very few parts fail to function properly when assembled into the final product;
otherwise, the assembled product is likely to malfunction.
Perfect precision is the ideal that is never quite realized, because its higher cost of
achievement must be balanced by the the lower costs of success.
It is possible to have acceptable accuracy and unacceptable precision, acceptable
precision with unacceptable accuracy, unacceptable accuracy and precision, and unacceptable
accuracy and precision acceptable. Obviously, the last condition is desirable in
making goods and providing services, provided that it can be achieved with an improvement
in costs or revenues. It is absolutely necessary to make interchangeable parts,
which are essential to automated, mass production of products. The history of
manufacturing is featured by continued improvements in
accuracy and precision of machines, tools and instruments, where they can be justified by
overall cost. Accuracy and precision were neglected by American automobile
manufacturers in their quest to keep manufacturing costs low, thus resulting in poor car reliability
and greater maintainance costs for American car buyers in the 1960s. Japanese cars
were better quality and more costly to manufacturers, but more reliable and less costly to their
buyers, so many Americans bought Japanese cars, which increased the revenues and profits
of the Japanese manufacturers and reduced the revenues and profits of the American
manufacturers. The American manufacturers got the message and emphasized
quality thereafter, which improved their competitiveness.
An adhesive is a compound that bonds two
surfaces together. Most early adhesives were glues made from rendered animal
products such as the horns, hooves and skins of ruminants, or plant gums and resins, or
bitumens, which are naturally occurring solid or liquid hydrocarbons, excluding coal.
are made of synthetic polymers that produce bonds that are stronger than the adhering
materials, so they have become less expensive substitutes for traditional fasteners, such as
nails, bolts, rivets, and screws. There are several types of adhesives:
These adhesives are a mixture of ingredients
(typically polymers) dissolved in a solvent.
Examples are glues and rubber cements.
As the solvent evaporates, the adhesive hardens. These adhesives are
typically weak and used mostly for household applications.
These adhesives are applied hot and allowed to harden as they cool. These
adhesives are popular for crafts because of their ease of use and the wide range of
common materials to which they can adhere.
Epoxy resins are the most common example of this kind of adhesive.
Reactive adhesives generally come in two separate containers that are mixed immediately
before application. The result is a reaction that solidifies the adhesive.
Reaction adhesives may also react with the surface of the materials to be stuck together by
chemical bonding, in contrast to sticking, the action of common glues.
A special kind of reactive adhesive is cyanoacrylate ("super glue") which reacts with moisture
in the air and therefore does not need any mixing before application.
Reactive adhesives are very strong and are used for high-stress applications, such as
attaching wings to aircraft. Because the strength of a reactive adhesive is a result of chemical
bonding with the surface material, reactive adhesives are applied in thin films. Reactive
adhesives are less effective when there is a secondary goal of filling gaps between the
Temporary adhesives are designed to repeatedly stick and unstick.
They have low adhesion and generally do not support much weight. They are
commonly used on paper, such as, bookmarks, informal notes and office supplies.
Although not usually classified as adhesives, solder and
cement also have adhesive qualities.
A generic word for any aerial vehicle. There are 4 categories of aircraft:
(1) lighter than air aerostats, e.g., airships, balloons
(blimps), (2) heavier than air gliders, which are motorless airplanes, (3) airplanes, and
(4) rotorcraft, e.g., helicopters and autogiros.
Airships achieve lift by a
large gondola filled with a gas that is lighter than air. Since air pressure varies
inversely with height, the pressure against the bottom of the gondola is greater than that at
the top of it, so it is pushed upwards. Engines drive propellers or fans turning inside
Gliders provide a cheap way to
convey large groups of people (e.g., technicians, aid workers, troops) to remote areas. They are
towed by airplanes and can land without a long airfield. Gliders operate on the same
principle as airplanes except that they do not have a motor that provides them with enough
forward thrust to achieve a sufficient pressure difference between the bottom and top of the
wing to achieve liftoff. This thrust is provided by an airplane towing it.
Airplanes have a motor to achieve
enough thrust to allow lift by a wing, called an airfoil, designed to maintain a higher
pressure on the underside than on the upperside. Flaps on the wings adjust the lift
capacity by increasing or decreasing their areas and inducing the wind to flow faster over and
under them. Ailerons on the wings enable the plane to bank or roll inwards while the
rudders turn it. Tailplanes keep the plane from dipping forward (pitch).
Elevators on the tailplanes achieve up and down motions.
An upright tail fin maintains horizontal left and right (yaw) stability.
It has a rudder to turn the plane left and right. The propeller, using the same principle as
the wing, but applied along the plane axis, pulls the plane at varying speeds.
Jet engines may or may not use propellers or fans to obtain thrust (propulsion).
Rotorcraft move in any direction, including sideways and
backwards. The main rotor provides lift, speed, and motion in any direction by
adjusting the blade angle (pitch). The
smaller tail rotor keeps the craft from spinning as a result of by the main rotor torque and
assists the main rotor in the desired direction.
The helicopter achieves flight by a
motor-driven rotor that acts like a wing or propeller in an airplane. It can move in any
direction, hover, and take off and land vertically, thus needing no airstrip.
uses a rotor that is activated by the wind, like a windmill, not by a motor. it requires a
motor-driven forward propeller and must achieve lift by first moving forward on land; therefore,
it cannot lift vertically nor hover like a helicopter.
- Alkali Metal
So called because their hydroxides (-OH) and carbonates
(-CO3) for alkalis (bases), they are
cesium, rubidium, and francium.
These metals are very soft (they can be cut with a knife), and good conductors of
electricity. Being chemically active, they are
never found free in nature. They react violently with water, so they must be stored in
containers under paraffin or in a vacuum.
A mixture of two or more
elements, at least one of which is a metal.
Brass is an alloy of copper and zinc.
Bronze is an alloy of copper and tin or other metals. Alloys are
made to have more useful properties than either of the alloying metals.
Alloys of metals and mercury are called amalgams.
A process to produce an
oxide film (coating) on metals and
alloys by electrolysis.
The metal to be treated is made the anode in an
electrolytic cell and its surface is
electrochemically oxidized. Anodization can improve certain surface properties, such
as corrosion resistance, abrasion resistance, hardness, appearance, etc. One metal
very often anodized is aluminum, on which all the
above properties improve.
Furthermore, since the surface film is porous, the aluminum metal can be colored by the
application of pigments or dyes in the pores.
The aluminum part to be anodized made the anode (positive electrode) in an
electrolytic cell. This aluminium is immersed in an
electrolyte consisting of an acid and water solution.
Sulphuric acid is used for relatively soft, easily dyed coatings and organic acids are used
for hard coatings. The temperature of the solution is controlled to give the desired
metallic properties. For example, at 20°C, a sulphuric acid anodizing solution
will give a soft, transparent clear, easily dyed coating, whereas at 5°C a hard, dense,
dull grey coating is produced (called hard anodizing).
DC (direct current) electricity is passed between the aluminium that is made the anode, the
electrolyte and a cathode, which is often lead.
When the current is applied, the water in the electrolyte breaks down and oxygen is deposited
at the anode. This oxygen combines with the aluminium to form an oxide that builds
on the oxide film always present on the aluminum surface.
The acid in the electrolyte tries to dissolve this oxide and produces a porous oxide film on the
aluminium surface. Coating thickness up to 25 microns is recommended for external
use. The oxide grains are hexagonal in shape and each grain contains a hexagonal
hole within it.
Once the required thickness of the oxide is obtained, the aluminium is removed from the
electrolyte and rinsed thoroughly to remove the acids from the pores in the film.
The oxide produced is porous and will accept or trap any material into its pores, which can be
advantageous or disadvantageous, depending on use. The advantage is the ease of
painting or dyeing. However, if a non-porous finish is desired, then the pores are
closed or the coating is "sealed". This is done by adding water (hydrolyzing) to the
oxide, which causes it to swell and close. The resultant film is then smooth, hard,
homogenous and transparent. The sealing process may be carried out in boiling
water, or in chemically enriched water at room temperature. Anodizing differs from
electroplating in that with electroplating, the material to be
coated is at the cathode rather than at the anode. However, they both use the same
- Aqua Regia
Aqua Regia ("royal water", or nitrohydrochloric acid) is a mixture of concentrated
nitric acid (HNO3) and concentrated hydrochloric acid
(HCl). The proportions, respectively, are 1 part to 3 or 4.
It's fame comes from its ability to dissolve gold and platinum (the once royal metals),
which each of its constituent acids cannot. The combination, however, releases
water and chlorine (Cl2), a powerful oxidizing agent, that forms an
ion with the metal in the solution.
An artificial canal for carrying water over some obstacle. It is used to supply
water to a populated area and to convey a
boat or barge over a
river, ravine, or structures, such as city buildings.
A substance that contracts the tissues or canals of the
body to diminish discharges, such as