4 On a Modification of Mance's Method of 



Dr. Lodge, in a very valuable and interesting paper, pub- 

 lished in the Philosophical Magazine for June, 1877, has 

 painted out two defects in this method. 



1. The balance of resistances being tested by the variation 

 of the current in AC, which will be small compared with 

 the current itself when the balance is nearly obtained, a 

 sensitive galvanometer cannot be employed ; and even the 

 sensitiveness of the coarse galvanometer generally used has 

 to be diminished by the intensification of the magnetic field 

 in which its needle moves, caused by the external magnets 

 used to bring the needle near its zero position. 



2. Although, when the resistances are balanced, altering 

 the resistance of BD does not directly change the current in 

 AC, it does change the current in AB. Now the electro- 

 motive force of a battery does, to some extent, depend upon 

 the strength of the current passing through it, so that if 

 the resistance in BD is altered the change in the electro- 

 motive force of the battery thereby produced will alter the 

 current in AC. 



In some cases the resistance of AB will also vary with 

 the strength of current passing through it. Dr. Lodge, 

 in his paper, says that if the resistance of a con d actor 

 depends on the strength of the current passing through 

 it, the resistance is not a definite magnitude, and 

 cannot be measured. I would, however, point out that, 

 while Ohm's law no doubt implies that the resistance of a 

 conductor is a fixed quantity independent of the strength of 

 current passing through it, yet changing the strength of the 

 current may produce a change in the conductor itself (as in 

 the temperature of a wire, the chemical composition of the 

 parts of a voltaic battery, the size and temperature of an 

 electric arc), whereby its resistance is altered. I imagine 

 that it is in this indirect way that change of current 

 strength affects electromotive force. 



Dr. Lodge, in the paper above referred to, describes a 

 modification of Mance's method, which consists in intro- 

 ducing a condenser in the branch AC, and employing a 

 special form of key whereby contact in BD is made for but 

 a very small portion of time. 



In order to obtain the greatest sensitiveness, the resistances 

 of AD and DC should be as great as possible, and the resist- 

 ance of the galvanometer and capacity of the condenser also 

 great. 



