162 White/wad and Hill — Measurement of Self -Inductance. 



V. 



Methods 7, 8, 9, 10, 11, and 12. 



Of the remaining zero-deflection methods which depend on a 

 ninety degree phase difference, little needs to be said. Methods 



7 and 8 are for determining — but involve mutual inductance 



\j 



and are intended to be used with doubly wound coils. For 

 accuracy greater than one per cent such coils are very undesir- 

 able on account of the electrostatic capacity, and should not 

 be used. Methods 9 and 10 are for comparing mutual induc- 

 tance with capacity. Method 12 was investigated by Mr. 

 Penniman. 



VI. 



Methods 15-21,.. 



The rest of the methods require at least two adjustments; first, 

 the Wheatstone bridge must be balanced with direct currents, 

 then adjustment made with alternating current so that the cur- 

 rent through the bridge arm is zero. Of these methods. 15 and 

 16 are for comparing two inductances, two capacities or induc- 

 tance with capacity. However, Method 15 requires after bal- 

 ancing the bridge with direct currents two simultaneous adjust- 

 ments with alternating current; and Method 16 requires many 

 resistances of known ratio. Method 17 is for the measure- 

 ment of — , but as Professor Rowland points out: "It is 



difficult to apply, as two 

 resistances must be ad- 

 justed and the adjust- 

 ment will only hold while 

 the current period re- 

 mains constant." These 

 methods did not seem to 

 be worthy of trial. 

 Method 18 is for the 



determination of — 

 M 



where L and M belong 



to the same coil. This 



method is simpler than 



most of the others and seemed rather more promising, so it 



was given a trial. The diagram of the method is shown in 



fig. 12. 



The bridge is first balanced with direct currents giving 



R'R,, =■ R'R'. 



M' 



12 



