196 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
zinc reflectors (1'3 metres from the dynamometer) used for the illumination of the 
scales, was found to produce no magnetic effect on the deflection. 
The “ leading in ” wires to the fixed coils were all wound non-inductively, and at 
the entrance to the coils the direction of the currents in the four sets of thicker wires, 
forming the small loops (visible in Plate l) were such that their resultant magnetic 
effect at the centre was completely eliminated. A magnetometer test provided a 
simple proof of this matter. 
The bifilar suspensions which constituted the leading in wires for the suspended 
coils formed, as will be seen from their dimensions given in § VI. (a), a long rectangular 
circuit of appreciable dimensions. It is, however, evident that this circuit extends only 
to the circumference of the suspended coils and is partly outside the fixed coils and 
partly inside. A calculation will show that the resultant effect from the suspensions 
is of a negligible order. The connections on the suspended coils made it possible to 
test this by reversing the direction of the current in the suspensions without changing 
its direction in the coils. No effect on our deflection could be detected. 
XII. The Electromotive Force of the Mean Weston Normal Cell. 
It has been explained that the electromotive force of the standard cell is given by 
the drop in potential across the two ohm standard when the current 4 i 2 passes through 
the resistance at a temperature of 23°'12 C. We now have all the factors. It has 
been shown that 
B = 56593-2, r = 40-1428, 
tan 6 = 0-139201,, -4G 1 ^ 1 -4G 6 .g 5 P' 5 -4G 7 c/ 7 P' 7 = 1216280. 
Hence by expression (4), [§ II. (£>)], i 2 in absolute amperes is given by 
i n a /56593‘2 x 0-139201, A . 107A0Q ■, , 
i 2 =10 \ f -- = 0 127023o absolute amperes. 
v 40-1428 x 1216280 F 
Hence the drop in potential across the standard two ohms, 2"00368 international 
ohms, is given by 
4 x 0'127023 3 x 2"00368 = 1 "01806 semi-absolute volts. 
This is equivalent to the electromotive force of our cell Pj or P 2 at 25°"03 C. Using 
the temperature formula 
E t = E 20 -0-000040(£ —20) —0-0000009 (£-20) 2 , 
we see that the electromotive force of our cell at 20° C. is 1 "01828 semi-absolute volts. 
It has been pointed out in § X. that these cells are about 30 microvolts lower than the 
value of the international “ Mean Weston Normal Cell,” hence our final result for the 
electromotive force of the Mean Weston Normal Cell is 
P01831 semi-absolute volts at 20° C. 
