Self-induction of Wires. 23 



slowly periodic or steady currents, it tends to readily pass 

 rapidly periodic currents, a property which is very useful in 

 telephony, as in Van Rysselberghe's system. 



On the other hand, the coil passes the slowly periodic, and 

 tends to stop the rapidly periodic, a property which is also 

 very useful in telephony. A very extensive apphcation of 

 this principle occurs in the system of telephonic intercommu- 

 nication invented and carried out by Mr. A. W. He? vi side, 

 known as the Bridge System, from the telephones at the vari- 

 ous offices being connected up as bridges across from ore to 

 the other of the two conductors which form the line. Whilst 

 all stations are in direct communication with one ar other, 

 one important desideratum, there is no overhearing, which is 

 another. For all stations except the two wlr'ch are in corre- 

 spondence at a certain time have electromagnets of high 

 inductance inserted in their bridges, which electromagnets will 

 not pass the rapid telephonic currents in appreciable strength, 

 so that it is nearly as if the non- working bridges were non- 

 existent, and, in consequence, a far greater length of bur ed 

 wire can be worked through than on the Sequence system, 

 wherein the various stations have their apparatus in sequence 

 with the line, whilst at the same time a balance is preserved 

 against inductive interferences. When the two stations have 

 finished correspondence, they insert their own electromagnets 

 in their bridges. As these electromagnets are used as call 

 instruments, responding to slowly periodic currents, we have 

 the direct intercommunication. Of course there are various 

 other details, but the above sufficiently describes the principle. 



As regards the property of the self-induction of a coil in 

 stopping or greatly decreasing the amplitude of rapidly 

 periodic currents, or acting as an insulation at the first 

 moment of starting a current, its influence was entirely over- 

 looked by most writers on telegraphic technics before 1878, 

 when I wrote on the subject (Journ. S. T. E. &E. vol. vii.). 

 A knowledge of the important quantity (R 2 + L 2 n 2S f , which 

 is now the common property of all electrical schoolboys 

 (especially by reason of the great impetus given to the spread 

 of a scientific knowledge of electromagnetism by the com- 

 mercial importance of the dynamo), was, before then, confined 

 to a few theorists. 



If the coil R, L, and the condenser Sx be in parallel, we have 



C =( S ^ + R^) V ' 



or 



V_ R+{L-S 1 (R 2 + L 2 n 2 )} 

 C~ (l-LS 1 n 2 ) 2 +(RS 1 n) 2 ' 



