346 Dr Searle, The comparison of nearly 
6. Practical example. The intercomparison of three coils 
is illustrated by the following results obtained by G. F. C. Searle 
and A. L. Hughes. Sub-standards each of nominally one ohm 
resistance were used. 
Comparison of A and B. 
aj=0, I/m=0, ~b,=2800,  1/b,=0-000357 
dg=O, 1/a,=0, 6, = 4800, 1/b,=0-000208. 
Thiel : 1 
(O00; 000208) = = +0-0002 
Hence ieeeuweo (0:000357 + 0:000208 ) Rr 000282 
and A—B=—AB x 0:000282= —0:000282 ohm. 
Comparison of A and C. 
m=O, 1/a,=0, ¢, = 4000, 1/¢e,=0°000250 
@2=785, 1 /d,=0°001274, c=o, 1/e.=0. 
ili yseeih i el 
Hence At 9% 0001274 = G+ 5 x 0:000250 
and A—C=ACx 0000512 =0:000512 ohm. 
Comparison of B and C, 
b, =2700, 1/b,; =0-000370, Cir 1/e,;=0 
bo= 760, 1/b,=0:001316, Cg=0, 1/e.=0. 
There : ; : el 
Hence Rta (0:000370 + 0:001316) = C 
and B-C=BCx0-000843 =0:000843 ohm. 
The value of B—C deduced from the two differences A—B and A —C'is 
B-C=(A-—C)—(A — B)=0-000512 + 0:000282=0:000794 ohm. 
Thus the two values of B—C only differ by 0:000049 ohm. 
§7. Intercomparison of fowr coils. When the student has 
sufficient time he may determine dzrectly, by the method of § 2, 
each of the six differences 
A-B, A-(C, A—D, B-C, B-D, C—D. 
When this has been done, it will be found that the six differences 
are not quite consistent. Thus, in the example recorded in § 6, it 
was found by direct comparison of B and C that B—C=0-000843 
ohm but that 
(A —C)—(A —B) = 0000794 ohm. 
We have, then, to decide how to combine the six results so as to 
obtain the most probable values of the three differences A — B, 
A-—C, A—D, it being supposed that each of the six observed 
ditferences has been found with the same care. 
The method employed to obtain the desired result is the 
method of least squares. 
