Wires
Electric currents are usually led from place to place, at will, by means
of conductors called _wires_. There are a great many kinds of wires,
each adapted to some special purpose.
Wires are usually covered with a material called an _insulator_, in
order to prevent the loss of electric current due to the wires coming
into contact with other bodies or circuits. Insulators are substances
which do not conduct electricity.
Wires which are _insulated_ by heavy braids of cotton fiber and then
impregnated with some compound, such as creosote, are called
_weather-proof_ wires, and are best adapted to outside service, where
they must be exposed to the action of the elements.
The wires used for interior wiring in buildings, etc., are usually
insulated with rubber, over which is placed a cotton braid to protect
the rubber.
Rubber cannot well be used as an insulator for all wires, although its
insulating value is very great, owing to the fact that it deteriorates
under many conditions.
Rubber-covered and weather-proof wires are made in a variety of
insulations. Some may have only one insulating layer, while others have
a great many. Different substances are used as insulators to adapt the
wire to some special purpose. Copper is usually the only metal used to
form the wire or conductor itself. The reason for this is that copper is
a better conductor than any other metal except those known as precious
metals, such as gold and silver, the cost of which prohibits their use
for such purposes. The wire may be solid, or made up of a number of
small conductors so that it is flexible.
The various combinations of insulating layers, together with either a
solid or a stranded conductor, have made possible a variety of
current-carriers, known as:
Theater or Stage Cable
Elevator Cable
Fixture Wire
Telephone Wire
Mining Cable
Feeder Cable
Brewery Cord
Heater Cord, etc.
depending upon the special use for which they were designed.
The wires which the young experimenter is likely to use in his work the
most are known as _magnet wires_, and are used for making
electro-magnets, coils, and various windings. Magnet wires may be
insulated with either silk, cotton, or enamel.
Silk-covered and cotton-covered wires may be obtained with either a
single or double covering.
Wires with a single covering of silk or enamel are used when it is
desirable to save space, for the covering of these two classes of magnet
wires is thinner than either the cotton or double-silk-covered wire, and
consequently they require less room for winding.
The size of the wire is indicated by its diameter, and in the United
States is measured by the Brown and Sharpe gauge, often indicated by the
term, "B. & S."
The preceding table shows the various sizes of wire of the Brown and
Sharpe gauge, and also several of their characteristics, such as weight,
resistance, etc.
Insulators
The covering placed over wires is not the only precaution taken to
insulate them, but in the case of permanent wiring they are usually
mounted on glass or porcelain supports.
[Illustration: Fig. 89.—Staples and Wooden Cleat used for running Low
Voltage Wires.]
Wires used for batteries, bells, telephones, etc., operated by batteries
and where the voltage is not over 20 volts, may be run under _insulated_
staples or wooden cleats inside of a building. If outside and exposed to
the weather, they should be mounted on suitable glass or porcelain
knobs.
[Illustration: Fig. 90.—Porcelain Insulators to support Electric Light
Wires.]
Electric-light wires for inside use are commonly supported by insulators
made of porcelain and known as cleats, knobs, and tubes according to the
shape.
Telegraph, telephone, and power lines are usually supported by glass
knobs or large porcelain insulators which screw on to wooden pins.
[Illustration: Fig. 91.—Glass Insulator Binding-Posts and Pin used to
support Telegraph and Telephone wires.]
Binding-Posts
Binding-posts are the most convenient device to make quick connections
between wires and other parts of electrical apparatus.
Binding-posts may be either made or purchased. Those which are purchased
are of course the best, and will add greatly to the appearance of any
instrument upon which they are mounted.
Several of the best-known types of manufactured posts are shown in
Figure 92.
[Illustration: Fig. 92.—Types of Binding-Posts.]
Figure 93 shows different ways of making simple binding-posts and
connectors from screws, washers, screw-eyes, and strips of metal. The
drawings are self-explanatory and should need no comment.
[Illustration: Fig. 93.—Home-made Binding-Posts.]
The screws and nuts obtainable from old dry cells are very convenient to
use for binding-posts and other similar purposes.
Switches and Cut-Outs
Switches and cut-outs are used in electrical work for turning the
current on and off.
If the experimenter chooses to make them himself, care should be taken,
to construct them in a strong and durable fashion, for they usually are
subjected to considerable use, with consequent wear and tear.
[Illustration: Fig. 94.—Binding-Post removed from the Carbon of a Dry
Cell.]
Several very simple home-made switches are illustrated in Figure 95.
[Illustration: Fig. 95.—Simple Switches. _A_, Single-Point Switch. _B_,
Two-Point Switch. _C_, Three-Point Switch. _D_, Five-Point Switch. _E_,
Lever with End Rolled up to form Handle. _F_, Lever with Handle made
from part of a Spool.]
The first one shown (_A_) has one contact, formed by driving a
brass-headed tack through a small strip of copper or brass.
The movable arm is a strip of copper or brass, rolled up to form a
handle at one end. The other end is pivoted with a brass screw. The
brass screw passes through a small strip of copper or brass having a
binding-post mounted on the end. A small copper washer should be placed
between the movable arm and the copper strip to make the switch work
more easily.
A somewhat similar switch is shown by _B_ in the same illustration, only
in this case a handle made from half of a spool is used, instead of
rolling up the end of the arm.
The other illustrations show how the same method of construction may be
applied to make switches having more than one "point" or contact.
No dimensions have been given for constructing these switches, because
it is doubtless easier for the young experimenter to use materials which
he may have at hand, and construct a switch of his own proportions. Only
one suggestion is necessary, and that is to bevel the under edges of the
arm with a file, so that it will slip over the head of the brass tack
more easily.
The switches shown in Figure 96 are capable of carrying heavier currents
than those just described, and more nearly approach the type used on
lighting and power switchboards.
The base may be made of wood, but preferably should be made of some
insulating substance such as fiber or slate.
[Illustration: Fig. 96.—Knife Switches.]
The patterns for the metal parts are shown in Figure 97. These are cut
from heavy sheet-brass or sheet-copper, and then bent into shape with a
pair of flat-nosed pliers.
The handle of the single-pole switch is driven on over the metal tongue.
The double-pole switch is almost a duplicate of the single-pole type,
but has two sets of levers and contacts, actuated by the handle, in
place of one.
[Illustration: Fig. 97.—Metal Parts for the Knife Switches.]
The ends of the blades to which the handle is connected are turned over
at right angles and a hard-wood cross-bar fastened between the ends. The
handle is fastened to the center of the cross-bar.
After the switch is assembled, bend the various parts until they "line
up" that is, are in proper position in respect to each other, so that
the blades will drop into the contacts without bringing pressure to bear
on either one side or the other of the handle in order to force the
blades into line.
Fuses
Fuses are used to prevent electrical instruments and wires from damage
due to too much current flowing through. When an electric current passes
through a resistance it produces _heat_.
A fuse is usually a short piece of lead or some alloy which melts at a
low temperature, and which is inserted in the circuit so that the
current must flow through it. If too much current flows through the fuse
it will become hot and melt, because of its low melting-point, thus
interrupting the circuit and shutting the current off until the cause
which occasioned the surplus current to flow can be ascertained.
Fuses are rated according to the amount of current which is required to
"blow" them out, and therefore are called 1, 3, 5, or 10 ampere fuses,
as the case may be.
[Illustration: Fig. 98.—Simple Fuses. _A_, Fuse-Block with plain Wire
Fuse. _D_, Fuse-Block with Mica Fuse in position.]
When a fuse burns out in a trolley car or in a light or power circuit,
it is because a greater amount of current is trying to pass than the
circuit can safely carry. If a fuse were not placed in such a circuit so
as to shut the current off before the danger point is reached, any
electrical device might become "burned out," or in extreme cases, the
wires become so hot as to cause a serious fire.
Figure 98 shows several simple forms of fuses which the experimenter may
easily make to protect the batteries, etc., from short circuits.
The simplest possible fuse consists merely of a small piece of lead wire
or a strip of thick tinfoil held between two binding-posts mounted upon
a wooden block.
The same form of fuse may be made from a strip of mica about two and
one-half inches long and one-half an inch wide.
A strip of thin sheet-copper is bent around the ends of the mica strip.
A piece of fuse wire is stretched between the two copper contacts and
fastened to each with a drop of solder. Fuse wire of any desired
ampere-carrying capacity can be obtained from most electrical supply
houses.
Such a fuse is held in a mounting as shown by _D_. The contacts are made
from sheet-copper or brass. They should spring together very tightly, so
as to make perfect contact with the copper ends on the mica strip.
Lightning-Arresters
Lightning-arresters are used to protect all wires which run into a
building from outdoors, especially telegraph or telephone wires, so that
static electricity due to lightning will not damage the instruments.
Lightning-arresters may be constructed in many ways and of different
materials, but there are only two types for which the average
experimenter will have any use.
[Illustration: Fig. 99.—Lightning-Arrester and Ground-Wire Switch.]
The arrester shown in Figure 99 is the type known as "lightning-arrester
and ground-wire switch." It is used principally on telegraph lines.
It consists of three pieces of sheet-brass about one-sixteenth of an
inch thick, and shaped as shown by _A_, _B_, and _C_ in Figure 100.
The metal pieces are mounted on a wooden block with a narrow space of
about one-thirty-second of an inch separating them.
[Illustration: Fig. 100.—Home-made Lightning-Arrester.]
The two outside pieces are each fitted with two binding-posts, and the
center triangular-shaped piece is fitted with one post.
A hole about one-eighth of an inch in diameter is bored between each of
the metal pieces.
Make a tapered metal pin which can be placed tightly in the holes, and
will make contact between the metal pieces.
The two outside line wires of the telegraph circuit are connected to the
outside metal pieces _C_ and _B_. _A_ is connected to the earth or
ground.
In case of a lightning storm, if the wires become charged, the small
space between the metal plates will permit the charge to jump across and
pass harmlessly into the ground.
If complete protection is desired, it is merely necessary to insert the
plug in one of the holes, and thus "ground" either wire or short-circuit
both of them.
[Illustration: Fig. 101.—Lightning-Arrester for Telephone Wires.]
The lightning-arrester shown in Figure 101 is designed for service on
telephone wires. It is an ordinary fuse provided with an arrester in the
shape of two carbon blocks about one inch square. The blocks rest on a
copper strip, and are held in place by a spring-strip connected to _B_.
The carbon blocks are separated by a piece of thin sheet-mica, of the
same size as the blocks.
The post, _B_, is connected to one of the telephone-line wires near the
point where it enters the building from outdoors. The post, _A_, is
connected to the instrument; _C_ is connected to the ground.
An arrester of this kind should be connected to each one of the
telephone wires.
If the line wires should happen to come into contact with a power wire,
there is danger of damage to the instruments, but if an arrester is
connected in the circuit such an occurrence would be prevented by the
blowing out of the fuse. If the lines become charged by lightning, the
charge can easily pass over the edge of the mica between the two blocks
and into the ground.