Signalling with Condensers. 427 



ment of contact with the battery become approximately constant 

 at any one part of the line — and, if there be no leakage, will attain 

 the same strength at every part of the circuit, including the bat- 

 tery and receiving instrument. The second method, first intro- 

 duced by Mr. Varley, and now in pretty general use on subma- 

 rine lines, is somewhat different. The end of the coils of the 

 receiving instrument, which in the first method is connected 

 with the earth, is now joined to one armature or inductive sur- 

 face of a so-called condenser, properly speaking an electrical 

 accumulator, the other armature of which is to earth ; or, which 

 comes to the same thing, the condenser is placed between the 

 line and the receiving instrument. As there is now no longer 

 a complete conductive circuit, no permanent current can flow 

 through the receiving instrument, or indeed through any part of 

 the line, if the insulation be perfect. 



Imagine the condenser to be a continuation of the cable, in 

 fact a length of cable having the same capacity as the condenser, 

 insulated at its further extremity, and the receiving instrument 

 connecting the main cable with its imaginary continuation, as 

 shown in fig. 1 ; where /is the battery at the sending-end of the 

 line, one pole of which is to earth, K a key for making contact 

 between its other pole and the cable A, and e the receiving in- 

 strument placed between A and the continuation B. 



Fig. 1. 



Then when contact is made at K it is evident that only so 

 much electricity will pass through e as will charge B up to the 

 potential of the further end of A. The current through e will 

 therefore be transient, rising to a maximum and then dying 

 away. This method of representation would be perfect if we 

 could neglect the resistance of the conductor inside B ; as, how- 

 ever, in practice the capacity of the condenser is only a fraction 

 of that of the line, there will be little difference due to this cause. 

 And if the capacity of B be very small, we may consider the flow 



