Nikola Tesla U.S. Patent 725,605 - System of Signaling Patent Wrapper Page 37

J J [ T T J web. Spec. selectively, and exclusively and without any danger of the signals or messages being disturbed, intercepted or interfered with in any way is the object of my present invention. Broadly stated, this invention consists in the combination of means for generating and transmitting two or more kinds or classes of disturbances or impulses of distinctive character with respect to their effect upon a receiving circuit, and a distant receiver which comprises two or more circuits of different electrical character or severally tuned so as to be responsive to the different kinds or classes of impulses, and which is dependent for operation upon the conjoint or resultant action of the two or more circuits or the several instrumentalities controlled or operated thereby. By employing only two kinds of disturbances or series. of impulses instead of one, as has heretofore been done to operate a receiver of this kind, I have found that safety against the disturbing influences of other sources is increased to such an extent, that I believe this number to be amply sufficient in most cases for rendering the exchange of signals or messages reliable and exclusive, but in exceptional instances, a greater number may be used, and a degree of safety against mutual and extraneous interference attained such as is comparable to that afforded by a combination lock. The liability of a receiver being affected by disturbances emanating from other sources as well as that of the signals or messages being received by instruments for which they arə not intended may, however, be reduced not only by an increas1 ed number of the cooperative disturbances or series of im1 pulses, but also by a judicious choice of the same and the order in which they are made to act upon the receiver. -3651