OP ARTS AND SCIENCES. 177 



that arising between the other two similarly placed coils whin 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 measurement. 

 Alexander Graham Bell employed a modification of Hughes's induc- 

 tion 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 arrangement. In 

 no form, however, of Hughes's induction apparatus can one obtain a 

 satisfactory minimum of tone in the telephone. There is never abso- 

 lute silence, and no two observers can obtain the same point at which 

 the sound seems to be a minimum. The failure to obtain this min- 

 imum 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 * substituted 

 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 comparison they employed was 

 a thin layer of mercury between disks of glass in a cylindrical reser- 

 voir. Preliminary investigations had shown the authors that a cer- 

 tain 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 subsequent 

 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 thai Messrs. 

 Overbeck and Bergmann had anticipated us,§ we were led to adopt 

 the following form of the instrument, which differed entirely from that 

 of these authors. Four coils were employed, as in the Hughes lorn. 

 of instrument. One of the coils in the telephone circuit was fixed 

 upon a horizontal axis which was at right angles to the axis of the 



* Annalen der Physik, xxxi., 1887, p. 792. 

 t Ibid., p. 812. 



| Maxwell's Electricity and Magnetism, vol. ii. § 668 et seq. 

 § Annalen der Physik, xxxvi., 1880, p. 783. 

 vol. xxiv. (h. s. xvi.) 12 



