400 ELECTRICAL MEASUREMENTS 



Therefore, when measuring an inductive coil a perfect balance 

 implies two things that the ohmic resistances are balanced 

 as in the ordinary Wheatstone bridge, and that the inductances 

 are in the ratio of the corresponding bridge arms. If there are 

 other than i z r losses in the arm X they appear in R x which in 

 this case is an equivalent resistance. 



With a bridge properly constructed, its coils being free from 

 inductance and capacity, it is thus possible to make a simul- 

 taneous measurement of the inductance and resistance to alter- 

 nating currents of a coil or piece of apparatus. 



To assist in carrying out the necessary adjustments in an 

 expeditious manner, it may be noted that non-magnetic conduct- 

 ors of small cross-section, used with currents of ordinary fre- 

 quencies, have practically the same resistance with alternating 

 as with direct current. Therefore, to save time, a preliminary 

 balance may be made with direct current, using the apparatus 

 as an ordinary Wheatstone bridge, thus satisfying the condition 



RM 

 RN 



Rx ~FT~ Rp- 



If the order of magnitude of the inductance under measure- 

 ment is entirely unknown, one may begin with R M = R N and 

 balance by varying R. It may be necessary to transfer this 

 resistance to the other side of the bridge by changing the lead 

 from c to e. Alternating is now substituted for direct current. 

 The detector will in general give an indication which must be 

 reduced to zero by adjusting the variable standard of inductance. 

 The chances are that on account of the limited range of the 

 standard, L P cannot be made of such a value as to obtain even a 

 minimum of sound in the telephone. In this case one notices 

 at which end of the scale the indication is the smaller, and then 

 alters the ratio so that the balance point will be thrown toward 

 the middle portion of the scale. The bridge is then rebalanced 

 for direct and for alternating currents. Two or three trials 

 may be necessary in order to obtain a good reading on L P . 

 If with R M = R N the indication is apparently the same at all 

 points of the scale, a large change should be made in the ratio; 



7? 7? 1 



for instance, to -^ = 10. If this does not give results, -~ = ^ 



PROPERTY OF ELE; 



PAP.UI TV ntr APOi icn c/^irii^ 



