RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
29 
vertical axis in the earth’s magnetic field. Currents are induced in the coil and these 
produce a deflection of a small magnet suspended at the centre. The dimensions of 
the coil and the time of its rotation being known, the resistance of the wire of 
the coil can be calculated. This method was also used by Lord Rayleigh and 
Prof. Schuster* in 1881 , by Lord Rayleigh! in 1882 , and by W. Weber} also in 
1882 . The results are given in Table I. 
In the method of the earth inductor due to W. Weber a coil is mounted on a 
vertical axis with the mean diametral plane in the magnetic meridian. In circuit 
with the coil is a ballistic galvanometer through which a quantity of electricity flows 
when the coil is quickly turned through half a revolution. The resistance of the . 
whole circuit can be calculated from the dimensions of the coils, the constants of the 
galvanometer, and the deflection produced by the half rotation of the coil. Methods 
based on this inductor principle have been used by F. Kohlrausch and G. Wiede¬ 
mann^ 
In a third method used by Kirchhoff, two coils, between which there is a mutual 
inductance, are joined up in series with a battery and galvanometer, and a resistance, 
R, joins the junction of the two coils to a point on the circuit between the galvano¬ 
meter and battery. The steady current deflection of the galvanometer is first 
observed and then the throw due to one of the coils being removed to a position in 
which the mutual inductance is zero. The dimensions of the coils and the constants 
of the galvanometer enable the resistance R to be calculated. Methods based on this 
inductive principle have been used by Rowland,|| Glazebrook, 1 F and Mascart, 
de Nerville, and Benoit.** Many of the developments of the method are of 
extreme importance, and, except for the essential principle being the same, there is 
little in common. Similar remarks apply also to the developments of other methods. 
The method adopted by Roiti ft and Himstedt}} is somewhat similar to that of 
Kirchhoff, but, instead of having to observe the deflection due to a single impulse, a 
constant deflection due to a series of impulses is obtained. The current through one 
of the fixed coils is made and broken n times per second, and the galvanometer 
circuit is made only on the make or break of the current. The method adopted by 
Guillet§§ belongs in part to this class and in part to the method suggested by 
Lippmann. 
* Rayleigh and Schuster, ‘Roy. Soc. Proc.,’ vol. 32, 1881. 
t Rayleigh, ‘Phil. Trans.,’ vol. 173, 1882. 
I Weber, ‘ Der Rotationsinduktor,’ 1882. 
§ G. Wiedemann, ‘Abh. der Berl. Ak.,’ 1884. 
|| Rowland, 1876, ‘Physical Papers,’ pp. 145-239; The Johns Hopkins Press, 1902. 
fi Glazebrook, ‘Phil. Trans.,’ vol. 174, p. 223, 1883. 
** Mascart, de Nerville, and Benoit, ‘Ann. de Chemie et de phys.,’ YI., p. 1, 1885. 
ft Roiti, ‘Nuovo Cimento,’ III., 15, 1884. 
\\ Himstedt, ‘Berichte der Naturforschenden Ges. zu Freiburg i. B.,’ Heft 1, 1886. 
§§ Guillet, ‘ Journ. de Physique,’ 8, pp. 471-477, 1899. 
