81 
ON THE ELECTRICAL PROPERTIES OF PURE SULPHUR. 
not to be recommended with bare ebonite, unless the pillar be arranged (as should 
always be the case) so that they can easily be dismounted and cleaned up with a 
cutting tool on a lathe ; mere washing or sand-papering does little good when very 
perfect insulation is required. 
Theory, which is well understood, indicates the following conditions for maximum 
sensitiveness :— 
(1.) The magnetic force per unit current should be a maximum. 
(2.) The magnetic moment of each member of the suspended system should be a 
maximum. 
(3.) The mirror should, if possible, be so good that its defining power is only limited 
by its size ; it should be optically perfect. 
(4.) The optical magnification should be a maximum. 
(5.) The astaticism of the suspended system should be a maximum. 
(6.) The field in which the suspended system moves should be zero when the 
astaticism is perfect, and the directional force entirely due to the unavoidable torsion 
of the suspending fibre. Since the astaticism is never perfect, and the field can never 
be zero, the torsion of the suspended fibre can only operate so as to reduce the sensi¬ 
tiveness, and must therefore be made as nearly zero as possible. 
It is obvious that the above conditions are partially incompatible, and, as a matter 
of fact, it is the last condition which provides a starting-point. 
The line of argument is this. In order to prevent changes of zero, we are compelled 
to use quartz threads. These threads are, however, very stiff. In order to prevent 
the stiffness being such as to seriously affect the sensitiveness, the threads must be 
very fine, and, consequently, the suspended apparatus very light. In order to go 
beyond this, it becomes necessary to determine to what point the optical sensitiveness 
can profitably be carried—or, what comes to the same thing, the limit of the angular 
value of the fluctuations of the zero. Under fixed conditions, the weight of the 
mirror will simply depend on the defining power required. The maker must take 
into account (I) the optical limit as to smallness, depending on the wave-length of 
light; and (2) the fact that the mirror must be thick enough neither to deform under 
its own weight, nor to deform owing to the freeing of internal stresses during its 
manufacture. Before exact theory can be applied to the construction of a galvano¬ 
meter it is, therefore, necessary to fix on the weight of the mirror, and though, with 
the experience we have now acquired with magnetically and thermally screened 
instruments, we can form an estimate of the minimum weight of the mirror, 
we could not do so before the instrument was constructed. It was necessary, there¬ 
fore, to proceed along the lines indicated by the general principles involved, and to 
make such alterations from time to time in the design as increased knowledge 
o Cr 
suggested. 
An alteration which is now in progress is the application of stronger material than 
glass to the preparation of the mirrors. Even hard glass, considered as a mineral is 
MDCCCXCVI.—A. M 
