Self-induction of Wires. 185 



In the course of these experiments I observed the upsettin & 

 of the resistance- and induction-balance by the presence of 

 metal in the neighbourhood of the coils, which is manifested 

 in an exaggerated form in electromagnets with solid cores. 

 So, having got the information I wanted in the first place, I 

 discarded the condenser method with its troublesome adjust- 

 ments, and, to study these effects with greater ease, went to 

 the equal- ratio method with the assistance that I had obtained 

 by the condenser method, the values of the inductances of 

 various coils, to be used as standards. 



" To use the Bridge to speedily and accurately measure the 

 inductance of a coil, we should have a set of proper standard 

 coils, of known inductance and resistance, together with a 

 coil of variable inductance, i. e. two coils in sequence, one of 

 which can be turned round, so as to vary the inductance from 

 a minimum to a maximum *. The scale of this coil could be 

 calibrated by (12a), first taking care that the resistance- 

 balance did not require to be upset. This set of coils, in or 

 out of circuit according to plugs, to form say branch 3, the 

 coil to be measured to be in branch 4. Ratio of equality. 

 Branches 1 and 2 equal. Of course inductionless, or prac- 

 tically inductionless, resistances are also required to get and 

 keep the resistance-balance. The only step to this I have 

 made (this was some years ago) .... was to have a number 

 of little equal coils, and two or three multiples ; and get 

 exact balance by allowing induction between two little ones, 

 with no exact measurement of the fraction of a unit." f 



Although rather out of order, it will be convenient to 

 mention here that although I have not had a regular induction- 

 box made (the coils, if close together, would have to be closed 

 solenoids), yet shortly after making these remarks, I returned 

 to my earlier experiments by calibrating the scale of the coil 

 of variable inductance. As it then becomes an instrument of 

 precision, it deserves a name ; and as it is for the measure- 

 ment of induction it may, I think, be appropriately termed 

 an Inductometer. Of course, for many purposes no calibra- 

 tion is needed. 



I found that the calibration could be effected with ease and 

 rapidity by the condenser method more conveniently than by 

 comparisons with coils. Thus, first ascertain the minimum 

 and the maximum inductance, and that of the coils separately. 

 Suppose the range is from 20 to 50 units (hundreds, thou- 



* Prof. Hughes's oddly named Sonometer -will do just as well, if of 

 suitable size and properly connected up. It is the manner of connection 

 and use that give individuality to my inductometer. 



t ' Electrician/ April 30, 1886, p. 490. 



Phil. Mag. S. 5. Vol. 23. No. 141. Feb. 1887. 



