30 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
Another method used by Weber is commonly called the method of damping. A 
magnet is suspended within a coil and set in oscillation (a) when the circuit is open, 
and (b) when the circuit is closed. The periods and logarithmic decrements are 
observed, and from a comparison of the results the resistance of the coil can be 
calculated. This method was used by Weber and with important modifications by 
Dorn,* * * § ** * * §§ Wild,! and Kohlrausch.J 
In the method due to Lorenz§ a metallic disc is rotated at a constant rate in a 
magnetic field produced by a current which circulates through a coil co-axial with 
the disc. The disc is touched at its circumference and centre by two wires, and the 
difference of potential is balanced against that at the extremities of a resistance, It, 
the current through which is the same as that circulating through the coil. When the 
mutual inductance of the coil and disc circumference, and the rate of rotation of 
the disc are known, the resistance It can be calculated. Methods based on this 
principle have been used by Lorenz, || Lord Rayleigh and Mrs. Sidgwick, 11 
Rowland and Kimball, ## Duncan, Wilkes, and Hutchinson, ft Jones, fl; and 
Ayrton and Jones.§§ 
Foster, ||[| and afterwards Lippmann, HIT suggested the use of a rotating coil, but 
contact with the extremities of the coil was made only at the moment when the 
induced voltage was a maximum. The induced voltage is balanced by that due to an 
external current (which may produce the field in which the coil rotates) through a 
known resistance. Observations by a method based on this principle have been 
carried out by Lippmann and Wuilleumier.*** 
The most recent determination of a resistance in absolute measure is due to 
A. Campbell.! ft In Campbell’s experiments two very nearly equal alternating 
currents in quadrature, taken from a two-phase alternator of sine-wave voltage, are 
passed through a resistance R and the primary circuit of a variable mutual 
inductance respectively. The ratio of these two currents is measured by passing 
them through equal resistances and comparing the deflections on an electrostatic 
* Dorn, ‘Wied. Ann.,’ 17, 1882, and 36, 1889. 
t Wild, ‘Mem. de l’Ac. des Sc. St. Petersburg,’ tome 32, Nro. 2, 1884. 
I Kohlrausch, ‘Abh. der bayr. Ak. d. W.,’ Bd. 16, 1888. 
§ Lorenz, ‘Pogg. Ann.,’ 149, p. 251, 1873. 
|| Lorenz, ‘Wied. Ann.,’ 25, p. 1, 1885. 
H Lord Rayleigh and Mrs. Sidgwick, ‘Phil. Trans.,’ vol. 174, p. 295, 1883. 
** Rowland and Kimball, ‘La Lumiere Electrique,’ vol. 26, pp. 188, 189, 477, 1887. 
ft Duncan, Wilkes, and Hutchinson, ‘Phil. Mag.,’ p. 98, 1889. 
Jones, ‘Electrician,’ p. 552, 1890. Also ‘B.A. Electrical Standards Reports,’ 1893, 1894. 
§§ Ayrton and Jones, ‘B.A. Electrical Standards Reports,’ 1897. 
HU G. Carey Foster, ‘B.A. Electrical Standards Reports,’ 1870. 
1111 Lippmann, ‘Comptes Rendus,’ 95, p. 1348, 1882. 
*** Wuilleumier, ‘Journal de Physique,’ 11, 9, p. 220, 1890. 
ttt Campbell, ‘ Roy. Soc. Proc.,’ 87, 1912. 
