ON STANDARDS FOR USE IN ELECTRICAL MEASUREMENTS. 123 
The keys Kk, and Ky were then reversed, and by the keys k, and k, 
one end of the 4 coil and one point on the wire P, Q, were connected 
through the galvanometer, and afterwards the two other ends. The 
riders were adjusted till there was no deflection of the galvanometer. 
The length of wire between the two riders had then a resistance of +); 
that of the } B.A. unit coil. 
Fie. 10. 
By means of the series of keys it was easy to repeat the observations, 
and to connect either end of the 4 coil with the wire. The resistance 
P, M, did not often change during the experiments, as the room was at 
a constant temperature; any change in it only caused a shifting of the 
position of the riders. In each experiment, after all the adjustments, 
the bath was well stirred, and everything left for half an hour. It was 
generally found that the riders did not require any readjustment. The 
battery was reversed, and all the coils moved. The latter never caused 
any effect ; sometimes the reversal of the battery caused a shifting of the 
two riders a millimetre or two in the same direction. Another reading 
was taken three or four hours after. 
The coils, $, 10, and 1, were in water baths, and their temperature 
remained the same for hours together. The temperature of the paraffin 
bath was not so constant; it was kept well stirred, and a thermometer 
divided to 0°-2 C. never showed any difference in the temperature at the 
different ends of the bath when the readings were taken. The thermo- 
_ meter employed was Kew-corrected; and the corrections given were 
verified by recent comparison with a platinum thermometer by Mr. 
Griffiths. 
Since the two standard coils employed were accurately in the ratio of 
10 to 1, the accuracy of the resistance measurement depended entirely 
on the value of the 4 B.A. unit. This was first made as nearly as possible 
3 but it was found that for the size of the wires measured (18 
.W.G.) this was too high a resistance ; it had therefore to be reduced. 
For the determination of its value there was cut ont in a-block of paraffin 
