204 Prof. W. Thomson and Mr. F. Jenkin un the True and 



The phenomena thus described and explained are entirely 

 different from any that could result from the galvanic polarization 

 supposed by Dr. Carl to account for them*. It is true that the 

 discharging earth-plate might become polarized by the discharge 

 in certain cases sufficiently to cause a slight reversal in the cur- 

 rent through the galvanometer coil, after the subsidence of the 

 violent discharge current through it. But in no case could the 

 whole quantity of electricity flowing in this supposed polariza- 

 tion current be more than a very small fraction of the quantity 

 which previously flowed in the true discharge current, of which 

 it is a feeble electro-chemical reflexion. Its effect on the gal- 

 vanometer needle must in every case be as nothing in com- 

 parison to the great impulsive deflection produced by the true 

 discharge current ; and there is no combination of circumstances, 

 as to size of the earth-plates, amount of the battery power, and 

 rapidity or sensibility of the galvanometer needle, in which the 

 cause supposed by Dr. Carl could possibly be adequate to explain 

 the phenomena described in Prof. Thomson's communication. 



In point of fact, all effects of polarization of the earth-plates 

 were extremely small in comparison w r ith the main currents 

 observed, which in the experiments on cables with one end 

 kept to earth, consisted of (1) the constant through-current, pro- 

 duced by a battery of 72 elements DanielFs in series ; (2) the 

 true discharge through the galvanometer to be observed instantly 

 after breaking the battery connexion of the end of the cable to 

 which the battery was applied, and making instead a connexion, 

 through the galvanometer coil, between the same end of the 

 cable and the earth; and (3) the " false discharge, " so called be- 

 cause it must have been often mistaken for the true discharge, 

 which almost necessarily escapes notice altogether when short 

 lengths of coiled cable are tested with slow galvanometer nee- 



* They are also different from any effects which could result from polari- 

 zation of the plate connecting the far end of the cable with earth — a cause 

 suggested by Prof. Wheatstone in a report published by the Committee 

 appointed by the Board of Trade to inquire into the Construction of Sub- 

 marine Cables. In support of his opinion, Prof. Wheatstone quotes some 

 experiments in which he could observe only the well-known effects due to 

 polarization, which on the short pieces of wire at his command quite over- 

 powered both the- true and false discharge. The current from the polarized 

 end of a cable is always in the direction of the true discharge when the 

 battery has been long enough applied : it is observed on both straight' and 

 coiled cables, and is capriciously variable. The details given in the present 

 paper show that the currents due to electro-magnetic induction, called 

 false discharge currents, are on the contrary always in the opposite direc- 

 tion to that of the true discharge, that they can only be observed on coiled 

 cables, and that they ^are in each case sensibly constant. The galvano- 

 meter used by Mr. Jenkin would not have been deflected half a degree by 

 the current from a polarized earth-plate at the end of cables from 300 

 to 500 knots in length. 



