THEORY OF WATTMETER 



47 



RCACTAMCl t.HT. 



4> = 6 ; aiid (3) the instrument reading is too high if <j> > 6, and the 

 rrmr increases with increase of 0. 



It may he pointed out that it is easier to render the error arising 

 from the self-inductance of 

 the fine-wire coil negligible in 

 wattmeters intended to he 

 used on a high than in those 

 for use on a low voltage cir- 

 cuit, since in the former case 

 the non-inductive resistance 

 may be made relatively 

 liigher, and so tan 6 may be 

 reduced. 



The main value of the 

 formulae obtained above lies, 

 not so much, perhaps, in the 

 fact that they enable us to 

 apply a correction, as in 

 showing under what conditions the correction may be safely neglected. 



FIG. 32. Effect of Reactance in Shunt Circuit 

 of Wattmeter. 



It 



-w- 



22. Effect of Capacity 



We may now consider the effect of capacity in the non-inductive 

 resistance. Imagine tliis resistance not subdivided as is generally 

 done but wound double in a single coil. Suppose next, for a moment, 

 that the middle point of the resistance, where the wire is doubled 

 back on itself, is cut. Then clearly the two halves of the winding 

 constitute a condenser, and if an alternating p.d. is maintained between 

 them, a definite capacity current or condenser current will flow 

 through the dielectric between them. 



Each half of the winding will at C 



every instant be at a practically 

 uniform potential. Let the cut ends 

 be now joined. We then have, in 

 addition to a capacity current, a 

 conduction current, and a continuous 

 drop of potential along the wire. 



Now, this drop of potential will obviously reduce the capacity 

 current to half the value which it had when the resistance was cut, 

 since the mean p.d. between the two halves of the resistance is now 

 only half the p.d. across its ends. The equivalent capacity is thus 

 half the true capacity, and the effect due to it is clearly the same as 

 if the resistance were devoid of capacity, but were shunted by a 

 condenser of capacity C equal to half the true capacity of the 

 resistance, as shown by the diagram of Fig. 33. 



FIG. 33. To illustrate Capacity Effect 

 in Wattmeter. 



