18 Trowbridge and Sheldon — Neutralization of Induction. 



by properly placing the coils in this circuit with reference to 

 those in the circuit through which the interrupted current 

 was passed, a balance could be obtained, or an imperfect mini- 

 mum of sound in the telephone, when the induction between 

 the sets of coils was neutralized In order to obtain a stand- 

 ard, Hughes employed a wedge of zinc, which was thrust 

 between one of the coils in the interrupted circuit and one of 

 the coils in the telephone circuit, in order that the mutual in- 

 duction between these coils might balance that arising between 

 the other two similarly placed coils when a coin or sheet of 

 metal was placed between these last mentioned coils. Other 

 devices have also been employed by various investigators who 

 have endeavored to use the apparatus for quantitative measure- 

 ment. Alexander Graham Bell employed a modification of 

 Hughes's induction balance for the detection of the presence 

 of a bullet in the human body. In the form employed by 

 him, one coil, which was a closely wound flat copper band, 

 was made to slide over a similar one by means of a screw, one 

 coil being placed in the telephone circuit and the other in a 

 circuit containing a current-breaker. The induction arising 

 from a similar pair of coils moved over a mass of metal like a 

 bullet could thus be neutralized by this sliding coil arrange- 

 ment. *In no form, however, of Hughes's induction apparatus 

 can one obtain a satisfactory minimum of tone in the telephone. 

 There is never absolute silence, and no two observers can 

 obtain the same point at which the sound seems to be a mini- 

 mum. The failure to obtain this minimum is thus a radical 

 defect in the instrument. It is doubtless very sensitive, but it 

 cannot be called a quantitative instrument. 



To remedy this defect, A. Overbeck and J. Bergmann* sub- 

 stituted an electro dynamometer for the telephone, and worked 

 out a method of obtaining the resistances of metals when they 

 are in the form of thin circular plates. The standard of com- 

 parison they employed was a thin layer of mercury between 

 disks of glass in a cylindrical reservoir. Preliminary investi- 

 gations had shown the authors that a certain relation existed 

 between the thickness and specific resistance and coefficient of 

 induction of metals in the form of thin disks, which were 

 placed between the coils of the induction balance. In a subse- 

 quent paper, f A. Overbeck gives the mathematical theory of 

 the induction balance, which in the main is Maxwell's theory 

 of current sheets applied to Arago's disk.;}: In employing the 

 instrument to measure the effect of change of temperature 

 on induction in copper plates, or, in other words, temperature 

 coefficients, in which we found that Messrs. Overbeck and 



* Annalender Physik, xxxi, 1 887, p. 792. f Ibid., p. 812. 



\ Maxwell's Klectricity and Magnetism, vol. ii, § 668, et seq. 



