ELECTRICAL MEASUREMENT BY ALTERNATING CURRENTS 299 



currents may fall into. Nearly all alternating currents generate elec- 

 tromagnetic waves which are so strong that currents exist in every 

 closed circuit with any opening between conductors in the vicinity. 



We eliminate this source of error by twisting wires together and other 

 expedients. But in avoiding one error, we plunge into another. For, 

 by twisting wires we introduce electrostatic capacity between them, 

 which may vitiate our results. Thus, in methods 23 or 24 for com- 

 paring mutual inductances, if there is electrostatic capacity between 

 the wires, a current will flow through the electrodynamometer in the 

 testing circuit and destroy the balance. 



Various expedients suggest themselves to eliminate this trouble, as, 

 for instance, the variation of the resistance A in the above, but I shall 

 reserve them for a future paper. I may say, however, that it is some- 

 times possible, as in method 12 for instance, to choose a method in 

 which the error does not exist. 



However, with the best of methods, much rests with the experimenter, 

 as errors from electromagnetic and electrostatic induction are added 

 to errors from defective insulation when we use alternating currents. 



These errors are generally less than one per cent, however, and intel- 

 ligent and careful work reduces them to less than this. 



The following methods generally refer by number to the plate on 

 which the resistances, etc., are generally marked. One large circle 

 with a small one inside represent an electrodynamometer. Of course 

 the circuit of the small coil can be interchanged with the large one. 

 Generally we make the smaller current go through the hanging coil. 



By the methods 1 to 14, we adjust the electrodynamometer to zero 

 by making the phase difference in the two coils 90. For greatest 

 sensitiveness, the currents through the two coils must be the greatest 

 possible, heating being the limit. This current should be first calcu- 

 lated from the impedance of the circuit, as there is danger of making 

 it too great. 



In the second series of methods, 15-26, the branch circuit in which 

 the current is to be is indicated by 0. 



Resistances in the separate circuits are represented by R, R', R t , etc., 

 and r, r', r t , etc. Corresponding self inductances and capacities in the 

 same circuits are L, L', L t , etc., and I, I', I,, etc., or C, C', C ',, etc., and 

 c, c', c t , etc. b = 27tn where n is the number of complete current waves 

 per second. 



The currents must be as heavy as possible, ^ ampere or more, and it 

 is well to make those that require a current of more than j-^ ampere of 



