False Discharge of a Coiled Electric Cable. 205 



dies. The through-current (1) was measured at the beginning 

 of the discharge experiments by introducing the galvanometer 

 into the circuit of cable and battery. Neither the whole 

 amount of the true and false discharges, nor the rapidly varying 

 strength of the current from instant to instant, could be distinctly 

 observed, because, the period of vibration of the galvanometer 

 needle, being about 4-g- seconds each way, was neither incompa- 

 rably greater nor incomparably smaller than the duration of the 

 current in either direction. Thus the back-flow, or true dis- 

 charge, which was of comparatively short duration, first gave 

 the needle an impulse to the left (let us suppose) ; but before 

 its natural, swing, from even an instantaneous impulse, could 

 have allowed it to begin to return, it was caught by the reverse 

 current of false discharge and turned and thrown to the other 

 side of zero through an angle to the right, which, except in the 

 cases of the longest lengths of cable experimented on, was much 

 greater than the angle of the first deflection to the left. It is 

 obvious from what has been stated, that the durations of these 

 deflections of the needle on the two sides do not even approxi- 

 mately coincide with the times during whicb the current flowed 

 in the directions of the true and false discharges respectively, 

 but that they depend in a complicated manner on the inertia of 

 the needle and the varying forces to which it is subjected. The 

 general character of the phenomena will be made sufficiently 

 clear by the following examples, which are quoted from letters of 

 Mr. Jenkin's to Prof. Thomson, of dates April 9 and April 22, 

 1859. 



Table I. 



Lengths of cable in 

 nautical miles*, — the 

 first being for the Dar- 

 danelles, and the other 



three, of a different 

 gauge, for the Alexan - 

 dria and Candia tele- 

 graph. 



Remote end of cable 

 kept insulated. 



Remote end of cable 

 kept to earth. 



First throw of needle. 



First observed throw 

 of needle. 



123 



137| 

 261^ 

 399i 



12 left 

 28i „ 



32 right 

 37 „ 

 31 „ 

 21 „ 



To explain the cause of the deflections to the right recorded 

 in the last column of this Table, the following observations were 

 made, with care that the first motion of the needle in either 

 direction, however slight or rapid, should not escape notice. 



* A nautical or geographical mile, or a knot as it is generally called in 

 nautical language, is taken as G08/ feet. 



