ire. 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.

CHAPTER XI

WIRELESS TELEGRAPHY

TELEGRAPHING WITHOUT WIRES.--Wireless telegraphy is an outgrowth of the
ordinary telegraph system. When Maxwell, and, later on, Hertz,
discovered that electricity, magnetism, and light were transmitted
through the ether, and that they differed only in their wave lengths,
they laid the foundations for wireless telegraphy. Ether is a substance
which is millions and millions of times lighter than air, and it
pervades all space. It is so unstable that it is constantly in motion,
and this phase led some one to suggest that if a proper electrical
apparatus could be made, the ether would thereby be disturbed
sufficiently so that its impulses would extend out a distance
proportioned to the intensity of the electrical agitation thereby
created.

SURGING CHARACTER OF HIGH-TENSION CURRENTS.--When a current of
electricity is sent through a wire, hundreds of miles in length, the
current surges back and forth on the wire many thousands of times a
second. Light comes to us from the sun, over 90,000,000 of miles,
through the ether. It is as reasonable to suppose, or infer, that the
ether can, therefore, convey an electrical impulse as readily as does a
wire.

It is on this principle that impulses are sent for thousands of miles,
and no doubt they extend even farther, if the proper mechanism could be
devised to detect movement of the waves so propagated.

THE COHERER.--The instrument for detecting these impulses, or
disturbances, in the ether is generally called a _coherer_, although
detector is the term which is most satisfactory. The name coherer comes
from the first practical instrument made for this purpose.

[Illustration: _Fig. 75._ WIRELESS TELEGRAPHY COHERER]

HOW MADE.--The coherer is simply a tube, say, of glass, within which is
placed iron filings. When the oscillations s