In Chapter XII. we showed how Dr. Hertz caused electric waves to pass through space and become visible by sparks across an air gap in a wire ring situated at a distance from the source of energy. The apparatus used, and termed an electric resonator, is in principle similar to that of the wireless telegraph. The minute sparks instead of idly passing across the air gap are made to traverse a "coherer" (to be afterwards more fully described). This "coherer" substantially consists of a resistance, preferably metal filings placed in series, with a battery and relay. Normally, the resistance is so adjusted that the battery current is not strong enough to operate the relay. A wire is led from one side of this coherer up into the air to intercept the Hertzian waves, the other side of the coherer is put to earth, or "grounded." When a wave strikes the air wire it sends a current through the coherer to ground (as before it sent a spark across the air gap), and this wave acts on the filings in its passage through them; in effect, to lower their resistance, so that the current is increased through the relay circuit and the relay armature is attracted to its magnet. The relay makes contact in the usual manner at the platinum points, and in its turn causes the local circuit, sounder, bell, or pen register to record the wave (or signal). After each wave the filings are in such state that to restore them to their former high resistance it is necessary to give the coherer a smart tap. This is generally accomplished automatically by means of an arm extending from the sounder lever, which strikes against the coherer each time the sounder armature is moved.
Fig. 74.
Figures 74 and 75 are diagrams of a simple circuit, Fig. 74 being the transmitting apparatus and Fig. 75 the receiving apparatus.
In Fig. 74 P P and S S are the primary and secondary of a Ruhmkorff coil, D two brass balls on the discharger, B the battery, K a key, in place of the usual contact breaker, which is either absent or screwed down; V a wire leading from one arm of the discharger up into the air, of a height varying with the results desired; G a ground plate in connection with the other discharger arm.
The coil condenser is left out of the diagram for sake of clearness; but, of course, is necessary to the operation of the apparatus.
In Fig. 75, C is the coherer, also called the Branly tube, or radio conductor; S a telegraph sounder, or electric bell; R a relay; R B and L B the relay battery and local battery, respectively; G a ground connection; M a resistance, or choke coil, and V a vertical wire, as in the transmitter; in fact, in the station set the same vertical wire answers for both transmitter and receiver.
