316 ELECTRICAL MEASUREMENTS 



The turning moment, and therefore the deflection, are pro- 

 portional to the mean product of the currents J 2 3 and / 56 and 

 this mean product multiplied by the resistance of the potential 

 circuit is the reading of the wattmeter. Using effective values, 

 power by wattmeter = 



7 JRr 2 (Ri + Re) + <**R P (L L + L c m) (L P m)-] 

 R P KD == 1 L [ Rp2 + ^ 2(Lp m)2 



The instrument should give the power in the load circuit 

 between the points 1 and 4, that is the power in the load, plus 

 the heating in the current coils. 



P L + He = IL*(RL + Re) 



I L W(L L +Lcm)(L P m) 



-] LL cm) P m 



~R P 



Eddy-current Errors. Another source of error, one not 

 amenable to calculation, may be found in instruments of faulty 

 design. It is that due to currents induced in masses of metal, 

 such as the frame of the instrument, or, in instruments of large 

 current capacity, in the current coils themselves, for these 

 coils must be made very massive in order to give the requisite 

 carrying capacity. 



This error should be reduced to a minimum by the design of 

 the instrument. In high-capacity instruments it will be nec- 

 essary to wind the current coil with a stranded conductor or its 

 equivalent, the strands being insulated from one another. 

 Great care must be taken in arranging them. The average 

 position of each strand in the cross-section should be the same 

 as that of every other strand, otherwise the heating of the coil 

 may change the current distribution and therefore the calibra- 

 tion of the instrument. 



In laboratory instruments intended for heavy currents, the 

 current coil may be made of a small and thin copper tube through 

 which there is a rapid circulation of water. 



To detect the presence of eddy-current errors in research in- 

 struments where the inductance of the potential-coil circuit is 

 compensated, the zero reading is brought to a noted point on 

 the scale and the compensation for the inductance of the moving- 

 coil circuit made as shown on page 313. The zero is then changed 



