164 MR. A. NORMAN SHAW: A DETERMINATION OF THE ELECTROMOTIVE 
(ix) The Kelvin-Varley is placed at the initial reading, and the galvanometer read 
at 330 seconds, and later at 390 seconds again. 
(x) The value of a galvanometer scale division is determined in terms of the 
Kelvin-Varley slide. 
(xi) The value of a small change in the shunt r 4 is determined in terms of the 
Kelvin-Yarley slide. 
We have thus obtained four measurements of the ratio for a given position of the 
coils chosen in such a way as to eliminate thermal and contact effects, and if the 
galvanometer deflections for 15, 20, 25, 30, 90, 330, and 390 seconds (which are all 
for the same arrangement of the circuit) are plotted, we can find by extrapolation the 
correction necessary to reduce the Kelvin-Varley readings taken 30, 60, 310, and 240 
seconds after the change, to the value for the ratio under the conditions of the 
operation (ii). 
In fig. 4 the type of behaviour at the commencement of the curve is shown, and 
in fig. 5 a curve is given which is typical of the curves taken with every set of 
operations. It will be seen how the corrections in column 8 of the table below can 
be determined with sufficient accuracy from such curves. 
Table IY. (p. 165) gives a summary of all the observations on the determination of 
the ratio of these currents. 
The first column gives the position of the coils. These are defined by A, B, C, D, 
E, F, G, H, I, J, K, L, M, N, O and P 
In positions 
of— 
Fixed coils 
are in 
position—- 
Suspended coils are 
placed— 
Remarks. 
With 
marked face 
to the— 
On— 
side of fixed 
coils— 
A, B, C, D 
II. 
4 
E, F, G, H 
I. 
— 
— 
(As defined for the main deflection expert 
I, J, K, L 
III. 
— 
— 
| ments (see § VII. («)). 
M, N, O, P 
IY. 
— 
— 
J 
A, E, I, M 
— 
West 
East 
B, F, J, N 
— 
West 
C, G, K, O 
— 
East 
East 
D, H, L, P 
— 
J? 
West 
The second column gives the arrangement of the circuit for the four readings on the 
Kelvin-Yarley slide, (l) corresponds to the reading in operation (iv) above; (2) to 
that in (v); (3) to that in (vii); and (4) to that in (viii). 
The slide reading is given in the third column ; the calibration correction has 
already been applied. 
The headings for columns (4) to (12) explain themselves. 
