RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
57 
the change in mutual inductance of the coils and the discs was worked out and found 
to be extremely useful when making the resistance measurements. The relation is 
as follows:—“ If, in the resistance measurements, t l is the mean of the initial 
temperatures of the marble cylinders, and t 2 the mean of the final temperatures, the 
mean mutual inductance (during the resistance observations) of all the coils and the 
two discs is the same as when the temperature of the cylinders is uniform throughout 
and equal to ^ + 0'42 (t 2 —t-^) other things being kept constant.” 
Fig. 11. Showing expansion of coils on marble cylinders at intervals from 5 to 35 minutes, when a 
current of 2 amperes is passed through them. 
Complete measurements of the coils have been made on three occasions. The first 
set of measurements was made in February and March, 1912, and two sets were 
made in April, 1913. In the first set 192 observations were made on each coil, 16 
observations being made in each of 12 axial planes 15 degrees apart. The turns of 
wire measured in any one plane were 1 cm. apart, and neighbouring strands were 
measured in succeeding planes, so that one measurement was made on every turn 
of wire. 
In the measurements made in 1913, six measurements were made in each of 16 
diametral planes 1 cm. apart, alternate sets of six measurements being made in axial 
planes 30 degrees apart, and the remaining measurements were made in planes 
midway between these. In addition, 16 observations were made in each of three 
axial planes 60 degrees apart, thus making 144 observations in all. Both pairs of 
gauge bars were used, and the difference found between them is identical within O’S^ 
with that deduced from the values already given. 
In the following table we give the mean diameters of the coils on the four cylinders 
as deduced from the measurements made in February and March, 1912, and April, 
1913 :— 
VOL. CCXIV.—A. 
I 
