SCIENCE IN THE INDUSTRIAL WORLD 



coil, the messages coming from the opposite direction 

 passing through with no effect. This arrangement, with 

 various modifications, came into general use shortly 

 after its invention. 



A somewhat similar device, which acts upon the prin- 

 ciple that currents of different intensities are not affected 

 by each other, and only act upon receivers of correspond- 

 ing intensity, is utilized for sending two or more messages 

 in the same direction, and at the same time, on one wire. 

 Supposing two sending instruments are given different 

 tensions, one high and one low, and two receiving instru- 

 ments are given corresponding tensions. If messages 

 are sent from the high-tension transmitter, such mes- 

 sages will be received by the high-tension receiver at 

 the end of the wire, no effect being produced upon the 

 low-tension receiver, which will be moved only upon the 

 operation of the low-tension transmitter. If both 

 transmitters are operated at the same time, however, 

 both high- and low-tension receivers will be affected, al- 

 though independently of each other. In this way it is 

 possible to send two messages at the same time. 



This system is utilized also in Elisha Gray's "Har- 

 monic Telegraph" for sending multiple messages. In 

 this, a number of separate magnetic vibrators, which 

 open and close the circuit at the rate of a certain number 

 of vibrations per second, are placed in connection with 

 the wire, with a corresponding vibratory receiver having 

 exactly corresponding vibrational periods to each of 

 the transmitting instruments. Such receivers will only 

 respond to the messages sent by the transmitter of simi- 

 lar vibrational period, so that no matter how many 



