302 THE ELECTRIC TELEGRAPH. 



one minute, or whatever time may be fixed upon since the 

 removal of the plug L L', and the plugs of U 2 rearranged, the 

 plug L L' is suddenly replaced, and the throw of the needle 

 observed. If the time which has elapsed is only a few 

 seconds, and the cable is well insulated, the deflection of the 

 discharge current is almost equal to that due to the charge ; 

 but when the interval is long, or the cable indifferently in- 

 sulated, the greater part of the charge is recombined through 

 the dielectric. 



The methods of comparing these two indications was sug- 

 gested by Dr. Siemens, and may be considered one of the 

 most important test methods in cable work. 



Dr. Siemens assumes the strength of the instantaneous 

 current which produces the swing to be proportional to the 

 sine of half the angle of deflection, or 



* = C sin. J 



C being a constant of sensibility of the instrument, and i the 

 current producing the deflection . For discharging, there- 

 fore, *\ being the returning current and \^ the swing which 

 it produces, we have 



'. = C sin - J- 



The difference of the two : i = i lt expresses the loss during 

 the interval between the two observations, or that portion 

 which has recombined through the insulating coating. The 

 charge being taken as unit, the loss, L, is, therefore, 



(h ' \L \L 

 sin. H __ sin. I sm. JL. 



i t , 2 2 , 2 







sm. J-. sin. L 



2 2 



Observations of two following charge currents may also 

 be taken as data for calculating the loss, instead of those of a 

 charge and a discharge. The plug L L' is removed from its 

 place and the key held down ; the plug is then replaced for 

 an instant, and again removed. On completing the circuit 



