Method of Measuring the Mutual Induction of two Coils. 85 



circuit, when it is of course equally in both wires, or not in 

 them at all ; but there is such a great gain in the theoretical 

 treatment of these problems by generalizing, that it is worth 

 while to point out the above restriction. 



Besides this case of equality of wires, which is precisely the 

 one that obtains in practice, there are other cases in which, by 

 proper proportioning of the electrical constants of the two 

 looped wires, the induction-balance is preserved ; and, simul- 

 taneously, we obtain transmission of signals as on a single wire. 

 Their investigation is a matter of scientific interest, though 

 scarcely of practical importance. 



I have yet to add investigations by the method of waves 

 (mentioned in Part IV.), by which method I have reached 

 interesting results in a simple manner. 



VII. Note on Prof. Carey Foster's Method of Measuring the 

 Mutual Induction of two Coils. By James Swinburne*. 



PROF. FOSTER suggests at the end of his paper f that 

 his method may be of use in dynamo work. Wishing 

 to get some arrangement for testing the induction through 

 any part of a dynamo, and for testing samples of iron for 

 Messrs. Crompton and Co., the writer devised an arrangement 

 which dispenses with the objectionable ballistic galvanometer. 

 It was tried roughly last summer, with the view of making a 

 permanent arrangement if the method worked well. 



The primary current was led through an electromagnet, A, 

 which represented the dynamo, and through one wire of a 

 double-wound coil, B, which represented a pair of coils of 

 known mutual induction. This coil was shunted by a wire, E, 



* Communicated by the Physical Society : read February 26, 1887. 

 t Phil. Mag. [5] vol. xxiii. p. 121 (February 1887). 



