﻿Mr. H. Wilde on a Property of the Magneto- electric Current. 61 



vided by the internal resistance of the battery; and from this 

 theory it is inferred that an electromotor, in order to overcome 

 a great external resistance, must itself possess a correspondingly 

 great internal resistance. A further consequence deduced from 

 this theory is, that the maximum useful effect of a given electro- 

 motor is obtained when the external and internal resistances are 

 equal. 



Now this mode of estimating the magnitude of an electric 

 current does not apply to the circuits on the armatures of my 

 machines. Taking for example the results obtained from the 

 quantity-armature of a 10-inch machine : — The dimensions of the 

 coil of this armature may be represented by a bar of pure cop- 

 per, 67 feet long, and having a sectional area of 1*6 square inch; 

 so that the resistance which this circuit presents to the passage 

 of a current, when compared with that of the liquids in a voltaic 

 battery, is practically null. When the coil is in full action it 

 will melt 15 inches of thin iron wire *035 of an inch in dia- 

 meter, or the same length of ^-inch iron rod with equal certainty, 

 and will electrolyze acidulated water in at least 16 voltameters in 

 series ; so that the resistance outside the circuit, whether esti- 

 mated by the 15 inches of thin wire melted or by the number 

 of electrolyzing-cells in series, is more than a hundred times as 

 great as that of the coil in which the current is generated. 



Moreover I have found that whenever a voltaic battery and a 

 magneto-electric machine will melt an equal length of wire, the 

 power which these electromotors have to overcome external re- 

 sistance, as measured by the number of voltameters in series, is 

 also equal. And, generally, the power of an electromotor (whether 

 voltaic or magneto-electric) to overcome external resistance is 

 directly proportionate to the length of wire which it will melt. 



From a consideration of these results, it will be seen that one 

 of the fundamental elements which enters into the theory of Ohm 

 is found wanting when that theory is applied to the estimation 

 of the magnitude of currents generated in circuits entirely me- 

 tallic. 



MM. Jamin and Roger, in a recent Number of the Comptes 

 Rendus of the Academy of Sciences*, have also pointed out the 

 discrepancy here referred to in the application of Ohm's theory 

 to magneto- electric circuits. 1 am, however, by no means pre- 

 pared to admit the correctness of the views advanced by these 

 physicists in their endeavours to reconcile the facts observed with 

 established theory ; besides which, other anomalies present 

 themselves when the customary formulas are applied to magneto- 

 electric circuits, a consideration of which must ultimately lead 



* Philosophical Magazine, October 1868. 



