On the Use of Mutual Inductometers, 497 



■or of special details o£ them may be got with greater ease 

 than at present, and any changes in rate from time to time 

 may similarly be detected. Unaffected by any personal 

 equation, the photographs have the further advantage of 

 being original documents which can be used at first hand by 

 any investigator. 



Flagstaff, Arizona, 

 Nov. 29, 1909. 



LIV. On the Use of Mutual Induct ometers. 

 By Albert Campbell, B.A.* 



(From the National Physical Laboratory.) 



§ 1. Introductory. 



§ 2. Modified Mutual Inductance Bridge. 



§ <f. Measurement of Effective Inductance. 



§ 4. Null Method in Iron Testing. 



§ 5. Tests of Current Transformers. 



§1. Introductory. 



IN a former paper | I have described arrangements, which 

 we may call mutual inductance bridges, by which self 

 inductances, even of very small amount, can be directly 

 measured %. I wish here to discuss further these and other 

 methods in which use is made of variable mutual inductances 

 or, to give them the more convenient name, mutual inducto- 

 meters. 



§ 2. Modified Mutual Inductance Bridge. 



In the equal arm bridge already discussed there is con- 

 siderable loss of sensitivity resulting from the insertion of an 

 auxiliary balancing self inductance in one of the arms. By 

 a modification of the arrangement of the bridge, however, 

 the use of the auxiliary coil is rendered unnecessary and the 

 whole apparatus becomes simpler and more efficient. Let us 

 first consider the more general case shown in fig. 1 where 

 the ratio arms are not equal. 



* Communicated by the Phvsical Society : read January 21, 1910. 



t Phil. Mag. Jan. 1908, p. 155. 



| I would mention here that many years ago Dr. Oliver Heaviside 

 investigated a very general case of inductance bridges (Phil. Mag. p. 173, 

 vol. xxiii. 1887). Most of the possible combinations are included in his 

 paper. 



Phil Mag. S. 6. Vol. 19. No. 112. April 1910. 2 K 



