per. A mirror attached to a vibrating
diaphragm reflects light from a lamp on to the strip, which is
automatically developed and fixed in chemical baths. The method of
moving the mirror so as to make the rays trace out words is extremely
ingenious. Messages have been transmitted by this system at the rate of
180,000 words per hour.

Chapter VII.

WIRELESS TELEGRAPHY.

The transmitting apparatus--The receiving apparatus--Syntonic
transmission--The advance of wireless telegraphy.

In our last chapter we reviewed briefly some systems of sending
telegraphic messages from one point of the earth's surface to another
through a circuit consisting partly of an insulated wire and partly of
the earth itself. The metallic portion of a long circuit, especially if
it be a submarine cable, is costly to install, so that in quite the
early days of telegraphy efforts were made to use the ether in the place
of wire as one conductor.

When a hammer strikes an anvil the air around is violently disturbed.
This disturbance spreads through the molecules of the air in much the
same way as ripples spread from the splash of a stone thrown into a
pond. When the sound waves reach the ear they agitate the tympanum, or
drum membrane, and we "hear a noise." The hammer is here the
transmitter, the air the conductor, the ear the receiver.

In wireless telegraphy we use the ether as the conductor of electrical
disturbances.[13] Marconi, Slaby, Branly, Lodge, De Forest, Popoff, and
others have invented apparatus for causing disturbances of the requisite
kind, and for detecting their presence.

The main features of a wireless telegraphy outfit are shown in Figs. 59
and 61.

THE TRANSMITTER APPARATUS.

We will first consider the transmitting outfit (Fig. 59). It includes a
battery, dispatching key, and an induction coil having its secondary
circuit terminals connected with two wires, the one leading to an
earth-plate, the other carried aloft on poles or suspended from a kite.
In the large station at Poldhu, Cornwall, for transatlantic signalling,
there are special wooden towers 215 feet high, between which the aerial
wires hang. At their upper and lower ends respectively the earth and
aerial wires terminate in brass balls separated by a gap. When the
operator depresses the key the induction coil charges these balls and
the wires attached thereto with high-tension electricity. As soon as the
quantity collected exceeds the resi