RESISTANCE BY A METHOD BASED ON THAT OF LORENZ. 
99 
(c) When the distance between the diametral planes is increased to 30"5794 2 cm., 
Mj is diminished by 3'19 ; and when the temperature of the coils is diminished to 
17°"5 C., M 7 is reduced by 3'06. 
( d ) When the temperature of the disc is reduced to 17° C., M 7 — M 2 is increased 
by 0'12. 
(e) Hence the value of M is 
M = Mj-Ma = 1158937—742M—3-17 —3-06 + 012 
= 115145-2 cm. 
(f) Mw is therefore 115145'2 x 17"3835 7 = 200163 5 . 
(7) Calculation of R:—- 
We have 
%Mn = 2tR {1 +(77 x 10- 5 )}. 
Hence 
R = 200163 5 /2 + 7 7 
= 100089 5 cm./sec. 
= 0"00100089 5 ohm. 
This value of R is on the assumption that the position of the mean diametral planes 
has been correctly estimated from the metrology measurements. To remove the 
assumption it is necessary to reverse the coils without interchanging, and this was done 
after 28 measurements of the O'OOl ohm standard had been made. The effect of 
reversal is very small and indicates that the assumed mean position of the diametral 
planes is very nearly correct. The mean result obtained in the two positions is taken 
as the value of R. When the coils Nos. 1 and 2 are used independently of Nos. 3 
and 4, an appreciable difference—about 4 parts in 100,000—is obtained on reversal of 
the coils. The conclusion is that the estimated position of the mean diametral planes 
of the coils Nos. 1 and 2 is incorrect by about 0"006 mm. The same is true for the 
coils Nos. 3 and 4. 
In Tables XX. and XXIII. we give the data relating to measurements of a resistance 
of nominal value 0"001 ohm, and Tables XXI. and XXII. give the results only of 
the measurements of a resistance of O'Ol ohm and an effective resistance of 0*002 ohm. 
The observed values in absolute measure and the values in international ohms (new 
N.P.L. Mercury Standards of Resistance) are given in Table XXIV. 
Probable Errors. 
The mean observational error of the results given in Table XX. is about 2 parts in 
100,000 for a single observation. This error includes all the errors arising from an 
inaccurate estimation of the distance apart of the coils, of variations in the speed, of 
an erroneous estimate of the mean speed, of faulty temperature observations, and the 
o 2 
