RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
81 
Section 19.— Effect of Electric Motor on the Mutual Inductance 
of the Coils and Discs. 
Originally it was our intention to employ a water turbine to drive the Lorenz 
apparatus, but there were many difficulties in the way of a satisfactory system, and 
finally we decided on an electric motor. Before coming to this decision we made a 
number of experiments on the effect of the presence of a large mass of soft iron on 
the mutual inductance of two coils each of 200 turns of wire and 24 cm. in diameter. 
The distance between the coils could be changed and the approximate mutual 
inductance between them was measured by Mr. A. Campbell. The iron employed was 
a mass built up of laminated sheets tightly pressed together, the dimensions of the 
mass being 35 x 18 x 8 cm. When the coils were parallel and their mean planes 7 cm. 
apart, the following values were obtained for the mutual inductance when the iron 
was placed on the common axis of the coils and at a distance d from their mean 
plane :— 
d in centimetres. 
M in microhenries. 
co 
4942-5 
70 
4942-6 
60 
4942-6 
39 
4945-2 
20 
4975-0 
co 
4942-6 
The distance apart of the coils was afterwards increased to 10 cm. and eventually 
to 20 cm., but in no experiment could the effect of the iron at a distance of 1 m. be 
detected by the mutual inductance measurements. The stray magnetic field produced 
by the motor is not a source of trouble and has no influence on our results; the only 
manner in which the motor can affect the resistance measurements is by its action, as 
a mass of iron, on the mutual inductance of the coils and discs. 
The field magnet of the motor is not large; its length parallel to the shaft is about 
30 cm., and its section at right angles to the shaft is about 1000 sq. cm. The 
distance from the centre of the motor to the centre of the nearest of the four coils is 
almost exactly 400 cm. 
Dr. G. F. C. Searle, F.R.S., who has taken great interest in our work and to whom 
we tender our thanks, has very kindly calculated the effect of a sphere of soft iron on 
the mutual inductance of a coil and circle, both of which are some distance away. If 
the radius of the sphere is 20 cm. (corresponding roughly to a mass of iron equivalent 
to our motor) and the coil and circle are of the same dimensions and the same distance 
from the sphere as in the Lorenz apparatus, the effect of the sphere on the mutual 
inductance appears to be about 1 part in 10,000,000 which is, of course, absolutely 
negligible. This value is in very good agreement with two values determined 
experimentally. 
VOL. ccxiv.—A. 
M 
