EFFECT OF EDDY CURRENTS 49 



section. But since the joint capacity of n equal condensers arranged 



in series with each other is -th that of one of them, we see that by 



n 



subdividing the resistance we have reduced the capacity to -^ of its 



n 



original amount. The subdivision of the resistance therefore affords 

 a simple means of reducing the capacity to any desired extent. 



23. Effect of Eddy Currents 



In most of the earlier wattmeters there was a source of error 

 whose importance was not realized, and whose very existence, in 

 fact, remained for a long time unknown. It is largely due to this 

 source of error that a strong prejudice was established against the 

 wattmeter, a prejudice which was only gradually overcome as the 

 construction of the instrument underwent improvement. The error 

 referred to is that due to eddy currents induced in any solid masses 

 of metal used in the construction of the instrument such as a metal 

 case, metal supports for coils, or even a heavy winding for the main 

 coil. The error due to eddy currents may be regarded as a twofold 

 one. In the first place, the field due to the main coil is distorted 

 by the action of the eddy currents, and so the torque acting on the 

 movable coil is altered; and secondly, there will be, as explained 

 in 11, an additional torque (which may correspond to either an 

 increase or a decrease of torque, according to circumstances) acting 

 on the movable coil, due to the presence in its neighbourhood of 

 large conducting masses. 



In order to avoid this source of error, the case of the dynamometer 

 type of wattmeter is invariably constructed of some insulating material, 

 such as ebonite or ambroin, and the same material is also employed 

 for the supports of the coils, etc. For large currents, the current coil 

 should be carefully stranded.* 



24. Effect of Mutual Inductance 



The errors arising from self-inductance, capacity, and eddy 

 currents are the only ones present in the zero type of dynamometer 

 wattmeter, in which the movable coil always occupies the same 

 position when a reading is taken. But since this type of instrument 

 requires a preliminary adjustment, it is not very convenient to use, 



* Another solution of the difficulty is to use a suitably designed current trans- 

 former, so that the current coil may be wound with thin wire. 



E 



