ELECTRICAL MEASUREMENTS. 289 



the chemical action has in very many cases been determined, 

 and when we know the quantity of heat, we have only to 

 multiply this by the mechanical equivalent of heat, and we 

 get the electro motive force ; for the electro-motive force is 

 the work done, or the amount of energy expended in the 

 circuit when unit of electricity traverses the conductor. 

 From the determination of the heat of chemical action, and 

 the mechanical equivalent of heat we have the data for 

 calculating the electro-motive force due to any given 

 chemical process, such as takes place in a galvanic 

 battery. 



Again, if the magnetic forces acting upon a conductor 

 which we move in a field of magnetic force are definitely . 

 known, and the motion of the conductor relatively to the 

 magnetic field is definitively known, we can again calculate 

 the quantity of work done in maintaining the motion, and 

 hence also the electro-motive force produced. Thus the 

 electro-motive force may be theoretically ascertained either 

 from the energy of the chemical action required to produce 

 the current, or from the work done in maintaining the 

 motion of conductors in the neighbourhood of magnets ; 

 though I cannot enter into the details in either case. 



When we have got a measurement of the current and of the 

 electro-motive force, the measurement of Resistance follows 

 from a comparison of the two. The electro-motive force 

 acting in any circuit divided by the strength of the current 

 which that electro-motive force produces, measures the 

 resistance of the circuit. That is the general principle of 

 all absolute methods of measuring the resistance of circuits. 

 There is here an apparatus by which such a comparison 

 can be made. This is a circular conductor which can rotate 

 about a vertical axis, and as it rotates a current is produced 

 in it by the earth's magnetic force. By suspending a little 

 magnet inside this little box at the centre of the circle the 

 strength of the current produced in the wire can be ascer- 

 tained by the deflection of the magnet. The deflection of 

 the magnet combined with the speed of the rotation and 

 the dimensions of the coil gives us the absolute resistance 

 of the conductor. This is the actual apparatus employed 

 in a very elaborate series of measurements by a Com- 

 mittee of the British Association, of which Professor Clerk 

 k Maxwell was the most active member. It was employed 

 ' 



