ELECTRICAL MEASUREMENT. 435 



resistance may be measured by velocity. Let the 

 velocity of the motion of this little bar, moved 

 upwards in the manner I have described, be such 

 as to produce in the galvanometer a deflection of 

 exactly 45. Then the velocity, which gives that 

 deflection, measures the resistance in the circuit, 

 provided always the galvanometer be arranged to 

 fulfil a certain definite condition as to dimensions. 

 The essential point of this statement is that the 

 result is independent of the magnitude of the 

 horizontal force of the earth's magnetism. The 

 galvanometer needle is directed by the horizontal 

 magnetic force of the earth. Let us suppose that 

 to be doubled ; the directing force on the needle 

 is doubled, but the inductive effect is doubled 

 also, and, therefore, the same velocity which causes 

 the needle of the galvanometer to be deflected 45 

 with one amount of magnetic force of the earth, 

 will cause the needle to be deflected by the 

 same number of degrees, with a different amount 

 of magnetic force of the earth. Thus, inde- 

 pendently of any absolute measurement of the 

 terrestrial magnetic force, we get a certain velocity 



F F 2 



