96 
MR. F. E. SMITH ON THE ABSOLUTE MEASUREMENTS OF A 
never considered them sufficiently serious to justify us in discarding the results. It is 
true that in two experiments we were not able to calculate the resistance ; in the first 
of these, the apparatus for recording the speed was not put in gear and so no record 
was made ; in the second, an interference with the battery connected to the motor 
produced a change of speed beyond our control and we had to abandon the experiment. 
In all other cases when a set of measurements was commenced, it was completed and 
the result is given in this paper. 
At a speed of about 17'4 revolutions per second, the product Mw is nearly 2x 10®. 
and the corresponding resistance is therefore about 0'002 ohm. We have already 
described the combination of coils which gives such an effective resistance, and we 
used the apparatus for its measurement on ten occasions. 
To measure a resistance of O'OOl ohm, the coils on cylinders Nos. 1 and 2 were placed 
in parallel with those on cylinders Nos. 3 and 4. The latter coils were found to be 
equal in resistance to the former within the limits of the errors of the measurements 
made, and a division of the main current into two parts, equal within about 1 in 3000 
could be ensured. In general, equality of the divided currents, within 1 per cent, 
would have been sufficient. The current through the standard resistance is now twice 
the mean A r alue of the current through the coils and the' resulting equation is 
= 2/lb When M n — 2x10®, it is O'OOl ohm. This arrangement was most 
convenient. A resistance of O'OOl ohm is more readily compared with 1-ohm standards 
than a resistance of 0'002 ohm, and as our standard of O'OOl ohm was of thick 
manganin strip, a current of 4 amperes could be passed through it without an 
appreciable heating effect. 
When the brushes are placed in series, a resistance of O'Ol ohm can be measured, as 
the equation 5t'Mn = ?R then holds good. Although this arrangement is very sensitive, 
a slightly greater pressure is required on each brush and this frequently produced 
trouble. 
A sample series of readings, taken on June 7, 1913, gives a good idea of the 
measurements involved in a single determination :— 
1 June 7, 1913. 
(l) Observations on invar line standard. Temperature = 15°'5 C. Lines 4-616. 
Length at 15°'5 C. = 30'6016 7 cm. 
Microscope readings (corrected). 
Left. 
Right. 
Diff. (L - R). 
f 1154-9 
946-6 
+ 208" 3 jj. 
Taken before readings on plugs < 
1135-0 
1028-0 
+ 207-0 
l 1133-4 
1025-4 
+ 208-0 
1 
f 979-4 
770-9 
+ 208-5 
Taken after readings on plugs < 
982-4 
774-1 
+ 208-3 
1 1035-1 
825-2 
+ 209-9 
