Nikola Tesla U.S. Patent 725,605 - System of Signaling Patent Wrapper Page 41

T J J T J justable resistance r3 and a relay R3. From the ahove description it will be readily seen that the relay R3 will be operated only when both contacts cl and c2 are closed. The apparatus at the sending station may be controlled 1 in any suitable manner, as for instance, by momentarily closing the circuit of the source S. Two different electric vibrations will be emitted simultaneously or in rapid succession, as may he desired, at each closure of the circuit. The two receiving circuits at the distant station each tuned to respond to the vibrations produced by one of the elements of the transmitter affect the sensitive devices al and a2 and cause the relays R1 and R2 to be operated and contacts 1 and 2 to be closed, thus actuating the receiver 1 or relay R3, which in turn establishes a contact c3 and brings into action a device a3 by means of a battery d4 included in a local circuit, as shown. But evidently, if through any extraneous disturbance only one of the circuits mf the messages may be secured. at the receiving station is affected, the relay R3 will fail to respond. In this way communication may be carried on with greatly increased safety against interference, and privacy The receiving station shown in Fig. 2 is supposed to be one requiring no return message, hut if the use of the system is such that this is necessary, then the two stations will be similarly equipped, and any well known means, which it is not thought necessary to illustrate here, may be resorted to for enabling the apparatus at each station to be used in turn as transmitter and receiver. In like manner the operation of a receiver, as R3, may be made dependent instead of upon two, upon more than two 655