ELECTRO-CHEMICAL EQUIVALENT OF SILVER, 
421 
The suspended coil. 
§ 13. This consisted of 242 turns of copper wire insulated with silk saturated with 
paraffin wax, and was wound upon an ebonite ring supplied by Messrs. Elliotts. 
The weight of the ring was 135 grms., and its section is shown full size in the adjoining 
figure (Plate 17, fig. 2). The weight of the wire was 440 grms., so that the total weight 
to be carried in the balance was about 575 grms. The mean diameter of the coil of wire, 
as determined from the inside and outside circumferences, was 8'090 inches; but it cannot 
be so determined with sufficient accuracy, and the result is not used in the calculation. 
It agrees perhaps about as well as could be expected with that deduced electrically by 
comparison with the large coil. 
The radial dimension of the section (2 h')= '9690 centim. 
The axial.(2F) = 1'3843 centims. 
The difficulties experienced in respect of the insulation, and the tests applied, have 
already been related (§ 11). 
The electrical comparison of radii (§ 14) gave for the ratio of the dynamometer 
radius A to that of the suspended coil a 
2-42113, 
whence 
a— 10'2473 centims. 
The mean radius thus determined is not necessarily that corresponding to the 
geometrical centre of the section, as it allows for any inequality in the distribution of 
the windings. 
The resistance of the coil is about 10^ ohms. 
Determination of mean radius of suspended cod. 
§ 14. 1'his quantity cannot be determined advantageously by direct measurement, 
but its ratio to that of the large coils can be deduced from the ratio of the galvano¬ 
meter constants of the coils, and this ratio can be accurately determined by the electrical 
method introduced by Bosscha.* 
It may be shown! that for all purposes we may take the mean radius and mean 
plane of a coil to correspond with the circle passing through the centre of density of 
the windings. If the windings are distributed with absolute uniformity, this point 
coincides with the geometrical centre of the section; otherwise there may be an 
appreciable distinction. The corrections of the second order, which in consequence of 
the finiteness of the section must be introduced in calculating the effects of the coil, 
have the same values as if the density of the windings were absolutely, instead of 
merely approximately, uniform. 
* Pogg. Aun., 93, p. 402,1854. 
f Camb. Phil. Proc., Feb. 12, 1883. 
