300 HENKY A. EOWLANB 



larger wire freely suspended in oil. A larger current can, however, be 

 passed through an ordinary resistance box for a second or two without 

 danger. A few fixed coarse resistances of large wire in air or oil with 

 ordinary resistance boxes for fine adjustment, are generally all that 

 are required. Special boxes avoiding electrostatic induction are, how- 

 ever, the best, but are not now generally obtainable. 



In some methods, such as 8, 9, 10, etc., we can eliminate undesirable 

 terms containing the current period by using a key which suddenly 

 changes the connections before the period has time to change much. 



In using twisted wire mutual inductances, methods 7 and 12 are 

 about or entirely free from error due to electrostatic action between 

 the wires. In all the methods this error is less when the resistance of 

 the coils is least and in 23 and 24 when A is least. In method 8 the 

 error is very small when the coil resistances and R are small and r great. 

 In this method with 1 henry and 1 microfarad the error need not 

 exceed 1 in 1000. Probably the same remarks apply to 9, 10, 11, also. 

 By suitable adjustment of resistances in the other method, the error 

 may be reduced to a minimum. It can, of course, be calculated and 

 corrected for. 



An electrodynamometer can be made to detect -OOC1 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 possibly 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 cur- 

 rent strength. Thus they have no fixed value. 



Closed circuits or masses of metal near a self inductance, dimmish 

 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 apparent 

 resistance, vary with the current period. They also introduce har- 

 monics. 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 resistance in 



