ELECTRICAL MEASUREMENTS 325 



Calculating what the absorption resistance should be for Troy and 

 ^ Elliott in series, from the absorption resistances of the two con- 

 densers when determined separately, it is equal to 10-26 ohms, which is 

 greater than the first and less than the last value above, showing that 

 the condensers were heating during the experiments. Calculating the 

 absorption resistance of Troy and -J Elliott in parallel in the same 

 way, it is equal to 2-209 ohms, which is less than the value afterwards 

 obtained by experiment for the same reason. 



The method was shown not to be based on any false supposition, by 

 substituting in place of the condenser a coil of known self-inductance. 

 When this was done the value of R^ as calculated from the other resist- 

 ances and the self-inductances should be the same as the actual ohmic 

 resistance of the circuit. 



This was tried with two coils P 2 and A and the agreement was re- 

 markably close, as seen in the next table. 



Coil P used in place of condenser in the E t circuit: 



Deduced value Actual value 



R" R,, R' r ofR, of R, 



474-9 487-8 758-2 5457- 77-86 77-8 



Coil A in place of condenser in the R, circuit: 



474-9 487-8 218-3 " 224-12 223-9 



In these experiments great care was taken that the measurements 

 of the resistances were performed immediately after the adjustment. 

 In this way the actual resistances at the time of the experiment were 

 obtained, and so the effect of the heating by the current was some- 

 what eliminated. 



Methods 26, 9 and 3 give good results, but the methods that gave 

 the most satisfaction were methods 12 and 6, method 12 being for the 

 comparison of two self-inductances and method 6 for the comparison 

 of a self-inductance with a capacity. These give some remarkable 

 results, the theory and deductions of the methods being as follows : 



Method 12. Zero Method for the Comparison of two 8 elf -Inductances 



Let the connections be made as in the figure where the hanging coil 

 and the fixed coils are in two distinct circuits. 



Let C<f iu etc. be the currents, A' and A" reversing commutators, 

 R", R and r the resistance of the different circuits, L" and L the self- 

 inductances, If the mutual inductance of the coils B\ and B 2 by which 

 it is placed. When a periodic electromotive force a m is applied to 

 A, B the quantity to be found is C^ C 8 cos ($ 3 0J where <p, fa 

 is the difference of phase. 



