ELECTRICAL MEASUREMENT. 243 



lifetime and also after his death the system of absolute measurement 

 in electrostatics. One most interesting result, brought out by Weber, 

 is that the electric resistance of a wire, in respect of electro currents 

 forced to flow from it, is to be measured in terms of certain absolute 

 units, which lead us to a statement of velocity in units of length per 

 unit of time, as the proper statement for the electro-magnetic measure 

 of the resistance of a wire. It would take too long to occupy your 

 attention on matters of detail if I were to explain minutely how it is 

 that resistance is to be measured by velocity. It seems curious, but 

 you will form a very general idea of it in this way. Suppose you have 

 two vertical copper bars and a little transverse horizontal bar, placed 

 so as to press upon those two bars. Let the plane of those two bars 

 be perpendicular to the magnetic meridian ; place then a little trans- 

 verse bar, like one step of a ladder, across the two vertical bars. Let 

 this bar be moved rapidly upwards ; being moved across the line of 

 the horizontal component of the earth's magnetic force, it will, accord- 

 ing to one of Faraday's discoveries, experience an inductive effect, 

 according to which one end of it will become positively electrified, and 

 the other negatively. Now, let the two bars upon which this presses 

 be connected together : then the tendency I have spoken of will give 

 rise to a current. That current may be made, as in Orsted's discovery, 

 to cause the deflection of a galvanometer needle. Now, you will see 

 how 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 attracted by the horizontal magnetic force of 

 the earth. Let us suppose that to be doubled ; the attracting force in 

 the needle is doubled, but the inductive effect is doubled also, and, 

 therefore, the same velocity which causes the needle of the galvano- 

 meter 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, 



