422 
LORD RAYLEIGH AND MRS. H. SIDGWICK ON THE 
For example, the galvanometer constant G L is related to the mean radius A 
(as above defined) and to the radial and axial dimensions of the section, 2 h, 2k, 
according to* 
G >-i 1+ *A> 
If, therefore, we can determine for two coils the ratio of galvanometer constants, it 
is a simple matter to infer therefrom the ratio of mean radii. 
In Bosscha’s method the two coils to be compared are arranged approximately in 
the plane of the magnetic meridian, so that their axes and mean planes coincide, and 
a very small magnet with attached mirror is delicately suspended at the common 
centre. If the current from a battery be divided between the coils, connected in such 
a manner that the magnetic effects are opposed, it is possible by adding resistances to 
one or other of the branches in multiple arc to annul the magnetic force at the centre, 
so that the same reading is obtained whichever way the battery current may circulate. 
The ratio of the galvanometer constants is then simply the ratio of the resistances in 
multiple arc. 
To obtain this ratio in an accurate manner, the two branches already spoken of are 
combined with two standard resistances so as to form a Wheatstone’s balance. Of 
these resistances both must be accurately known, and one at least must be adjustable. 
The electromagnetic balance is first secured by variation of the resistance associated 
with one of the given coils, which resistance does not require to be known. During 
this operation the galvanometer of the Wheatstone’s bridge is short-circuited. 
Afterwards the galvanometer is brought into action, and the resistance balance is 
adjusted. The ratio of the galvanometer constants is thus equal to the ratio of the 
known resistances. The two adjustments may be so rapidly alternated as to eliminate 
any error due to changes of temperature in the copper wires. 
The above comparison was carried out for each of the two coils of the dynamometer, 
and the coil wound on the ebonite ring, called for shortness the ebonite coil. On 
account of the smallness of the latter some care is necessary in the adjustments, which, 
however, do not require to be described in detail. It will be sufficient to refer to the 
description of the adjustments when the ebonite coil was suspended, and to mention 
that the errors arising from maladjustment (all of course of the second order) could 
hardly affect the final ratio by more than TotoTTo• The length of the magnet was 
inch, and the error due to neglecting it could not exceed xoaToo- To the magnet 
was attached a light silvered glass mirror, such as are employed in Thomson’s galva¬ 
nometers, and it was protected from air currents by a glass cell. The readings were 
taken by observing the motion of a spot of light thrown upon a scale in the usual way. 
The electrical connexions are shown in the adjoining figure (Plate 17, fig. 3). The 
current from a large Daniell cell A, after passing the reversing key B, divides itself 
* See Maxwell’s ‘ Electricity,’ § 700. 
