False Discharge of a Coiled Electric Cable. 



207 



Table III. 



I 



] Lengths of 

 Red Sea 

 cable. 



Remote end of length used kept insulated. 

 Discharge from electrification of 36 cells. 



Remote end of length used kept to earth. 

 True and false discharge from electrifica- 

 tion and current of 7'2 cells. 



First throw. 



Recoil. 



Excess of 

 first throw 

 above recoil. 



First throw. 



Recoil. 



Excess of 

 recoil above 

 first throw. 



312 nautical 



miles 



546 „ 



858 „ 



J20 . left 

 2% „ 

 35 „ 



19 right 



27 „ 

 14 „ 



i 



21 



1 i left 

 H ., 

 17 „ 



18 right 

 15 „ 

 22 „ 



16f 

 5 



Col. 1. 



Col. 2. 



True dis- 

 charge. 



Col. 3. 



Inertia of 

 needle. 



Col. 4. 



Effect of du- 

 ration of dis- 

 charge. 



Col. 5. 



True dis- 

 charge. 



Col. 6. 



" False dis- 

 charge "and 

 inertia. 



Col. 7. 



"False dis- 

 charge," or 

 effect of elec- 

 tro-magnetic 

 induction. 



The great increase of the numbers in column 4, for the 

 longer portions of cable, illustrates the fact first demonstrated by 

 Prof. Thomson in 1854*, that, when undisturbed by electro- 

 magnetic induction, the discharge of a cable takes place at a rate 

 inversely proportional to the square of the length. The duration 

 of the discharge, which, when the remote end is kept insulated, 

 is probably much increased by electro-magnetic induction, must 

 be very considerable in the case of the 858 miles length, to pro- 

 duce so great a diminution as 21° in the recoil, from a throw 

 of 35°, on a needle whose period of vibration was 4^ seconds. 

 The diminution of 1° from the throw of 20°, as observed in 

 the case of the 312 miles length, may be to some considerable 

 proportion of its amount due to resistance of the air, although, 

 as this is probably scarcely sensible on a single swing of the 

 needle, it may be supposed that it is chiefly the effect of the 

 duration of the discharge current. From column 7 it is clear 

 that nearly all trace of the electro- magnetic influence would 

 be lost sight of in comparison with the greater effect of true 

 discharge, in the method of experimenting that was followed, if 

 applied to lengths exceeding 1000 knots, in a coil or coils of 

 similar dimensions to those actually used; while for the 546 

 knots, and shorter lengths, the effect of electro-magnetic induc- 

 tion is greater than that of the true discharge. It is remarkable 

 that the effect of electro-magnetic induction is absolutely greatest 

 for the shortest of the three lengths. These relations between 

 the different lengths must of course, according to the explanation 

 we have given, depend on the plan of coiling, whether in one coil 

 or in several coils, and on the dimensions of the coil or coils, as 



* Proceedings of the Royal Society, 1855 ; and Phil. Mag. vol. xi. p. I4f>. 



