Signalling with Condensers. 431 



By comparison with the arrival-curve for the currerd at the 

 remote end when to earth, we see that, broadly speaking, it takes 

 about four times as long for the potential to nearly attain its 

 maximum when the end is insulated as it takes for the current 

 to nearly attain its maximum when the end is to earth. Thus, 

 when the end is to earth, the current reaches 98 per cent, of its 

 maximum strength in 20 a; and when the end is insulated, the 

 potential reaches 98 per cent, of its maximum in 80 a. This 

 relation does not hold good throughout the whole extent of the 

 curves ; but there is a general similarity. We may conclude that 

 signalling by means of an electrometer connected with the insu- 

 lated end of a cable would be much slower than the ordinary 

 plan of a galvanometer or recording instrument worked by the 

 current. 



Fig. 3 represents J from £ = to t = S0a calculated from 



equation (10), and is closely the same as the arrival-curve for 



dv 

 the current in condenser signalling. It will be seen that j 



reaches its maximum in 7 a. 



100. 



90 

 80 

 70 

 60 

 50 

 40 : 













Fig. 3, 



















































































































































































































30' 



























20 



























10 



















60 



The strength of the current will of course depend on the 

 capacity of the condenser, and will be proportional thereto so 

 long as it is small compared with the capacity of the line. As, 

 however, an increase in the capacity is equivalent to lengthening 

 the line, the maximum strength of the current will not be so 

 soon reached with the larger capacity : although the signals will 

 be stronger, they will be more retarded. Hence the best capa- 

 city to be used on any line, which should theoretically be as 

 small as possible, must be determined by the delicacy of the 

 instrument and the battery-power employed. When the capa- 

 city of the condenser is one seventeenth part of that of the line, 

 the maximum strength of a signal is about one tenth of the per- 



