H. A. Rowland — Electrical Measurement. 435 



may be reduced to a minimum. It can, of course, be calcu- 

 lated and corrected for. 



An electrodynamometer can be made to detect "0001 ampere 

 without making the self inductance of the suspended coil more 

 than '0007 henrys or that of the stationary coils more than -0006 

 henrys, the latter coil readily sustaining a current of ^ amperes 

 without much heating. 



An error may creep in by methods 1-14 if the current 

 through the suspension is too great, thus heating it and possi- 

 bly twisting it. This should be tested by short circuiting the 

 suspended coil or varying the current. For the zero method 

 it is eliminated by always adjusting until there is no motion 

 on reversing the current through one coil. 



Inductances containing iron introduce harmonics and vary 

 with current strength. Thus thev have no fixed value. 



Closed circuits or masses of metal near a self inductance, 

 diminish it, and increase the apparent resistance which effects 

 vary with the period. Short circuits in coils are thus detected. 



Electrolytic cells act as capacities which, as well as the ap- 

 parent resistance, vary with the current period. They also 

 introduce harmonics. The same may be said of an electric arc. 



An incandescent lamp or hot wire introduces harmonics into 

 the circuit. 



Hysteresis in an iron inductance acts as an apparent resist- 

 ance in the wire almost independent of the current period, and 

 does not, of itself, introduce harmonics. The harmonics are 

 due to the variation of the magnetic permeability with the 

 amount of magnetization. 



Electric absorption in a condenser acts as a resistance vary- 

 ing with the square of the period, the capacity also varying, as 

 I have shown above. 



In general any circuit containing resistances, inductances 

 and capacities combined acts as a resistance and inductance or 

 capacity, both of which vary with the current period, the square 

 of the current period alone entering. For symmetry the 

 square of the current period can alone enter in all these cases 

 and those above. 



Hence only inductances containing no iron or not near any 

 closed metallic circuits have a fixed value. The same may be 

 said of condensers, as they must be free from electric absorp- 

 tion or electrolytic action to have constants independent of the 

 period. There is no apparent hysteresis in condensers and 

 the constants do not apparently vary with the electrostatic 

 force. 



The following numbers indicate both the number of the 

 method and the figures in the plate, p. 437. 



