HIGH TENSION APPARATUS, CONDENSERS, ETC.
INDUCTION.--One of the most remarkable things in electricity is the
action of induction--that property of an electric current which enables
it to pass from one conductor to another conductor through the air.
Another singular and interesting thing is that the current so
transmitted across spaces changes its direction of flow, and,
furthermore, the tension of such a current may be changed by
transmitting it from one conductor to another.
LOW AND HIGH TENSION.--In order to effect this latter change--that is,
to convert it from a low tension to a high tension--coils are used, one
coil being wound upon the other; one of these coils is called the
primary and the other the secondary. The primary coil receives the
current from the battery, or source of electrical power, and the
secondary coil receives charges, and transmits the current.
For an illustration of this examine Fig. 70, in which you will note a
coil of heavy wire (A), around which is wound a coil of fine wire (B).
If, for instance, the primary coil has a low voltage, the secondary
coil will have a high voltage, or tension. Advantage is taken of this
phase to use a few cells, as a primary battery, and then, by a set of
_Induction Coils_, as they are called, to build up a high-tension
electro-motive force, so that the spark will jump across a gap, as shown
at C, for the purpose of igniting the charges of gas in a gasoline
motor; or the current may be used for medical batteries, and for other
purposes.
[Illustration: _Fig. 70._ INDUCTION COIL AND CIRCUIT]
The current passes, by induction, from the primary to the secondary
coil. It passes from a large conductor to a small conductor, the small
conductor having a much greater resistance than the large one.
ELASTIC PROPERTY OF ELECTRICITY.--While electricity has no resiliency,
like a spring, for instance, still it acts in the manner of a cushion
under certain conditions. It may be likened to an oscillating spring
acted upon by a bar.
Referring to Fig. 71, we will assume that the bar A in falling down upon
the spring B compresses the latter, so that at the time of greatest
compression the bar goes down as far as the dotted line C. It is obvious
that the spring B will throw the bar upwardly. Now, electricity appears
to have a kind of elasticity, which characteristic is taken advantage of
in order to increase the efficiency of the induction in the coil.
[Illustration: _Fig. 71._ ILLUSTRATING ELASTICITY]
THE CONDENSER.--To make a condenser, prepare two pine boards like A,
say, eight by ten inches and a half inch thick, and shellac thoroughly
on all sides. Then prepare sheets of tinfoil (B), six by eight inches in
size, and also sheets of paraffined paper (C), seven by nine inches in
dimensions. Also cut out from the waste pieces of tinfoil strips (D),
one inch by two inches. To build up the condenser, lay down a sheet of
paraffined paper (C), then a sheet of tinfoil (B), and before putting
on the next sheet of paraffined paper lay down one of the small strips
(D) of tinfoil, as shown in the illustration, so that its end projects
over one end of the board A; then on the second sheet of paraffine paper
lay another sheet of tinfoil, and on this, at the opposite end, place
one of the small strips (D), and so on, using from 50 to 100 of the
tinfoil sheets. When the last paraffine sheet is laid on, the other
board is placed on top, and the whole bound together, either by wrapping
cords around the same or by clamping them together with bolts.
[Illustration: _Fig. 72._ CONDENSER]
You may now make a hole through the projecting ends of the strips, and
you will have two sets of tinfoil sheets, alternately connected together
at opposite ends of the condenser.
Care should be exercised to leave the paraffine sheets perfect or
without holes. You can make these sheets yourself by soaking them in
melted paraffine wax.
CONNECTING UP A CONDENSER.--When completed, one end of the condenser is
connected up with one terminal of the secondary coil, and the other end
of the condenser with the other secondary terminal.
[Illustration: _Fig. 73._ HIGH-TENSION CIRCUIT]
In Fig. 73 a high-tension circuit is shown. Two coils, side by side, are
always used to show an induction coil, and a condenser is generally
shown, as illustrated, by means of a pair of forks, one resting within
the other.
THE INTERRUPTER.--One other piece of mechanism is necessary, and that is
an _Interrupter_, for the purpose of getting the effect of the
pulsations given out by the secondary coil.
A simple current interrupter is made as follows: Prepare a wooden base
(A), one inch thick, six inches wide, and twelve inches long. Upon this
mount a toothed wheel (B), six inches in diameter, of thin sheet metal,
or a brass gear wheel will answer the purpose. The standard (C), which
supports the wheel, may be of metal bent up to form two posts, between
which the crankshaft (D) is journaled. The base of the posts has an
extension plate (E), with a binding post for a wire. At the front end of
the base is an L-shaped strip (F), with a binding post for a wire
connection, and the upwardly projecting part of the strip contacts with
the toothed wheel. When the wheel B is rotated the spring finger (F)
snaps from one tooth to the next, so that, momentarily, the current is
broken, and the frequency is dependent upon the speed imparted to the
wheel.
[Illustration: _Fig. 74._ CURRENT INTERRUPTER]
USES OF HIGH-TENSION COILS.--This high-tension coil is made use of, and
is the essential apparatus in wireless telegraphy, as we shall see in
the chapter treating upon that subject.