ELECTEICAL MEASUREMENTS 317 



tude, the difference of potential between the terminals of the coil 

 might be great. If the connections under these circumstances were 

 made with double wire, as is customary, a great error was introduced 

 due to the electrostatic capacity of the leads. The error was sometimes 

 as much as 7 per cent (see method 24). This error could be shown to 

 be due to the electrostatic action of the leads by shifting a resistance in 

 circuit with the coil in question from one end of the double wire to 

 the other . The effect of this was to still further increase the difference 

 of potential between the leads, and this increased the error. Experi- 

 ments of this character showed the necessity of using open leads and 

 open resistances having little or no capacity in all cases in which the 

 coils experimented on and the resistance boxes used in their determina- 

 tion have a current of any considerable magnitude passing through 

 them. In several of the following methods constancy of current was 

 necessary. This was accomplished by various means that will be de- 

 scribed in their actual application. 



METHODS 



The methods that were tried were 25, 26, 9, 3, 12 and 6 described in 

 this Journal, December, 1897. 2 



Method 25. Method of equal deflections. Absolute method for the 

 determination of self-inductance or capacity in terms of electromagnetic 

 units. 



In this method the hanging coil is shunted off the fixed coils circuit, 

 and this with a non-inductive resistance in circuit with the hanging 

 coils is made the same as that of a certain inductive resistance in cir- 

 cuit with the hanging coil. The connections are made as in the Figs. 

 1, 2, where C e ibt , C r 1 e*' M +*i), C^^+W are currents. R, R', r, resist- 

 ances. They represent the entire resistance of their respective branches. 

 L represents self-inductance of the coil by which it is placed. The 

 outer circle in Fig. 1 represents the fixed coils and the small circle the 

 hanging coil of the electrodynamometer. In Fig. 2 the terminals of 

 the fixed and hanging coils are represented by F and H. D is a revers- 

 ing commutator. K is a key to send the current first through the 

 inductive and then through the non-inductive resistance. & = Z-xn, 

 n = complete alternations per sec. This is the general notation adopted 

 throughout the article. 



2 Phil. Mag., January, 1898. 



