False Discharge of a Coiled Electric Cable. 203 



connexion with the insulated pole of the battery, was suddenly 

 removed from the battery and put in connexion with the earth 

 through the coil of a galvanometer) Prof. Thomson and Mr. 

 Fleeming Jenkin remarked "that the deflections recorded in these 



Connexions used by Mr. Jenlrin. 



B. Battery. 



C. Cable. 



E„ Es, E 3 . Earth. 

 G. Galvanometer. 

 a, b, c. Three terminals of key d. 



experiments were in the contrary direction to that which the true 

 discharge of the cable would give ;" and at Prof. Thomson's 

 request " Mr. Jenkin repeated the experiments, watching care- 

 fully for indications of reverse currents to those previously noted. 

 It was thus found that the first effect of pressing down the key 

 [to throw the cable from battery to earth through galvano- 

 meter] was to give the galvanometer a deflection in the direction 

 corresponding to the true discharge current, and that this was 

 quickly followed by a reverse current generally greater in degree, 

 which gave a deflection corresponding to a current in the same 

 direction as that of the original flow through the cable. 



"Professor Thomson explained this second current, or false 

 discharge, as it has since been sometimes called, by attributing 

 it to mutual electro-magnetic induction between different parts of 

 the coil, and anticipated that no such reversal could ever be 

 found in a submerged cable. The effect of this induction is to 

 produce in those parts of the coil first influenced by the motion 

 of the key, a tendency for the electricity to flow in the same 

 direction as that of the decreasing current flowing through the 

 remoter parts of the coil. Thus, after the first violence of the 

 back flow through the key and galvanometer, the remote parts 

 of the cable begin, by their electro -magnetic induction on the 

 near parts, to draw electricity back from the earth through the 

 galvanometer into the cable again, and the current is once more 

 in one and the same direction throughout the cable." 

 P2 



