THE ELECTRIC TELEGRAPH. 



It may be shortly stated that the production of such signs 

 depends on the power of the agent transmitting the current to 

 transmit, suspend, intermit, divert and reverse it at pleasure. 

 These changes in the state of the current take place for all 

 practical purposes simultaneously upon all parts of the conducting 

 wire to whatever distance that wire may extend, for although 

 strictly speaking there is an interval, depending on the time which 

 the current takes to pass from one point to another, that interval 

 cannot in any case exceed a small fraction of a second. 



107. Although there is some discordance in the results of 

 experiments made to determine the velocity of the current, they 

 all agree in proving it to be prodigious. It varies according to 

 the conducting power of the metal of which the wire is composed, 

 but is not dependent on the thickness of the wire. On copper 

 wire, its velocity, according to Professor Wheatstone's experiments, 

 is 288000 miles ; and according to those of MM. Fizeau and 

 Gonelle, 112680 miles per second. On the iron wire used for 

 telegraphic purposes, its velocity is 62000 miles per second, 

 according to Fizeau and Gronelle ; 28oOO according to Professor 

 Mitchell, of Cincinnati ; and about 16000 according to Professor 

 Walker of the United States. 



108. It is evident therefore that the interval which must 

 elapse between the production of any change in the state of the 

 current at one telegraphic station, and the production of the same 

 change at any other however distant, cannot exceed a very 

 minute portion of a second, and since the transmission of signals 

 depends exclusively on the, production of such changes, it follows 

 that such transmission must be practically instantaneous. 



176 



