434 II. A. Rowland — Electrical Measurement. 



However, with the best of methods, much rests with the ex- 

 perimenter, 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 intelligent 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 electrodyna- 

 mometer. Of course the circuit of the small coil can be inter- 

 changed 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 calculated 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 cir- 

 cuit in which the current is to be is indicated by 0. 



Resistances in the separate circuits are represented by R R' 

 R, etc. and r r' r, etc. Corresponding self inductances and 

 capacities in the same circuits are L 1/ L, etc. and I V I, etc. or 

 C C ; C, etc. and c c' c, etc. b=2wn where n is the number of 

 complete current waves per second. 



The currents must be as heavy as possible, T \ ampere or 

 more, and it is well to make those that require a current of 

 more than T ^ ampere of 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 ordi- 

 nary resistance boxes for fine adjustment, are generally all that 

 are required. Special boxes avoiding electrostatic induction 

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



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

 desirable 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 resist- 

 ances 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 suita- 

 ble adjustment of resistances in the other method, the error 



