ALTERNATING CURRENT MEASUREMENTS. 5 1 



the metal disk are shown in Fig. 40. The disk DD rotates in 

 front of the three magnet poles ABB of a laminated iron struc- 

 ture as shown. The lugs BB are wound with coarse wire 

 through which flows the total current i that is delivered to 

 the receiving circuit, and the lug A is wound with fine wire 

 which is connected across the supply mains. 



The driving torque exerted by this device would be exactly 

 proportional at each instant to ei if the following two conditions 

 were exactly satisfied, namely: (a) If the magnetic flux in the 

 lugs B and B were exactly proportional at each instant to i, 

 and (b) if the electromotive force e across the supply mains were 

 exactly balanced at each instant by the electromotive force 

 induced in coil A by the changing flux through the lug A, 

 which would be the case if the resistance of coil A were negli- 

 gibly small. The action of the device will therefore be explained 

 on the assumption that these two conditions are satisfied; in 

 fact they are approximately satisfied in a well designed induc- 

 tion meter. 



The changing flux* through lug A produces not only an 

 electromotive force in coil A which is equal to e at each 

 instant, but this flux also induces a proportional electromotive 

 force in the disk along the dotted circles, and this electromotive 

 force produces a proportional current in the disk along the dotted 

 circles. This current in the disk, which is proportional to e, in 

 flowing under the lugs BB, causes the flux under BB to push 

 on the disk with a force which is proportional to the current (ke\ 

 in the disk and proportional to the flux (k'i), under BB. The 

 force acting on the disk is therefore proportional to ei at each 

 instant. The average driving torque is therefore proportional 

 to average ei, and, by using permanent magnets for damping 

 the motion of the disk, the disk is made to run at a speed pro- 

 portional to the driving torque. 



* The changing flux in lugs BB produces current in the disk, which current flows 

 under lug A. The average force due to this combination is also proportional to the 

 average power delivered to the receiving circuit. 



