Fig. 75.
The coil to be used may be from two inches of spark upwards, dependent upon the distance the signals have to travel. The relay battery may be two cells of dry battery, the local battery as much as is desired to operate the bell, sounder, or pen register receiving the signals. Presuming the apparatus set up and adjusted, and designating the transmitter as Station A and the receiver as Station B, the operation will be as follows: A pressure and release of key K sends an impulse of current through the primary P, inducing a current in S, which manifests itself by a spark between the discharger balls at D. An electric wave is released, which, starting from V, Station A, meets in its passage V of Station B. Travelling along this wire to the ground, it finds two paths—through C or R. As the choke coil deters it from passing through the relay, it finds passage through C and so to ground.
The Coherer.
Many forms of this apparatus are in use, but as yet no definite design can be recommended for specific purposes. The most general mode of construction is that of the Branley Coherer, as shown in Fig. 76.
Fig. 76.
It consists of a glass tube, 2 inches long by ¼ inch inside diameter, furnished with well-fitted metal plugs at each end, to which connections are made. These plugs can be slid in and out of tube for adjustment, the gap between them being loosely filled with fine metal filings. The metal used varies, according to the operator's preference, the most generally adopted being pure nickel for both plugs and filings. Another mode of construction for purely experimental use is to merely cork the ends of the tube and pass the wires through these corks into the filings, ensuring, however, good contact between wires and filings. Marconi's favorite form is a glass tube two inches long with silver plugs, each one-quarter inch long, in each end, intervening space being partially filled with a mixture of nickel and silver filings. These plugs are then adjusted to as close as one-twenty-fifth of an inch, and the whole apparatus exhausted of air either by means of a leading-in tube or by placing coherer in a vessel from which the air can be drawn. As a rule, coherers containing air become less sensitive after continued use.
Carbon Coherer.
Pointed carbon rods can be inserted in the tube instead of metal, and carbon dust substituted for the metal filings; but this form is suitable only for special purposes. It is very delicate in its action, but somewhat uncertain.