74 
ME. F. E. SMITH - ON THE ABSOLUTE MEASUREMENTS OF A 
Table XI. gives the constants used and the results obtained in one of the calculations 
dealt with in Table X. and gives also the dimensions of the Lorenz coils and brush 
contact circles at 20°'0 C. Table XII. gives (l) the differences of dimensions of the 
coils and standard, and (2) the mutual inductance M x of each coil and the contact 
circle nearer to it. 
Table XI. 
Coils on cylinder No.— 
Standard. 
1. 
2. 
3. 
4. 
. A = radius of coil in centimetres . 
17-9419 
17•9403 7 
l7-9408o 
17 * 9427 0 
17■9433 0 
x = length of coil in centimetres . 
16-0015 
16-0018 
16-0014 
16-0054 
16-0054 
a = radius of circle in centimetres 
d = distance of mean diametral "| 
26-7870 
26-791 
26-791 
26-780 
26-780 
plane of coil from the circle > 
in centimetres .... 
15-2892 5 
n = number of turns. 
16-0015 x12 
192 
192 
192 
192 
Mi = mutual inductance of coil 1 
and circle.J 
28976-54 
We summarise below our knowledge of the changes of M x with changes in the 
dimensions of the coils and disc when the distance of the mean diametral plane of the 
coil from the brush contact circle is equal to 15‘2892 5 cm. 
Change in Mi. 
1. Increase in radius of a coil by 10//.. +2'995 
2. Increase in radius of contact circle by 10//. 
3. 
Increase in length of coil by 10/x, the position of the 
mean diametral plane remaining constant, and the 
number of turns increased so that there may still 
be 12 turns per centimetre . 
V 
J 
-Q'0099 
+ U940 
4. Correcting factor to reduce the mutual inductance 1 
to a coil of exactly 192 turns is.J 
16/length of coil in 
centimetres. 
The values of M x given in Table XII. hold good only when the mean diametral plane 
of the coil is 15'2892 5 cm. from the circle. For other slightly different distances 
we may readily calculate the corrections from the data given in Tables IX. and X. 
The mutual inductance of the coils on cylinders Nos. 1 and 2 and the brush contact 
circle No. 1 is 57939 "7 when the distance between the'mean planes of the coils is 
30"5785 cm., and the corresponding values for the coils on cylinders Nos. 3 and 4 is 
