s at perfect command; the results at different
distances being very easily compared with each other either by experiment,
or, as they are inversely as the squares of the distances, by calculation.

1186. The Coulomb balance electrometer requires experience to be
understood; but I think it a very valuable instrument in the hands of those
who will take pains by practice and attention to learn the precautions
needful in its use. Its insulating condition varies with circumstances, and
should be examined before it is employed in experiments. In an ordinary and
fair condition, when the balls were so electrified as to give a repulsive
torsion force of 100 deg. at the standard distance of 30 deg., it took nearly four
hours to sink to 50 deg. at the same distance; the average loss from 400 deg. to
300 deg. being at the rate of 2 deg..7 per minute, from 300 deg. to 200 deg. of 1 deg..7 per
minute, from 200 deg. to 100 deg. of 1 deg..3 per minute, and from 100 deg. to 50 deg. of 0 deg..87
per minute. As a complete measurement by the instrument may be made in much
less than a minute, the amount of loss in that time is but small, and can
easily be taken into account.

1187. _The inductive apparatus._--My object was to examine inductive action
carefully when taking place through different media, for which purpose it
was necessary to subject these media to it in exactly similar
circumstances, and in such quantities as should suffice to eliminate any
variations they might present. The requisites of the apparatus to be
constructed were, therefore, that the inducing surfaces of the conductors
should have a constant form and state, and be at a constant distance from
each other; and that either solids, fluids, or gases might be placed and
retained between these surfaces with readiness and certainty, and for any
length of time.

1188. The apparatus used may be described in general terms as consisting of
two metallic spheres of unequal diameter, placed, the smaller within the
larger, and concentric with it; the interval between the two being the
space through which the induction was to take place. A section of it is
given (Plate VII. fig. 104.) on a scale of one-half: _a, a_ are the two
halves of a brass sphere, with an air-tight joint at _b_, like that of the
Magdeburg hemispheres, made perfectly flush and smooth inside so as to
present no irregularity; _c_ is a connecting piece by which the apparatus
is joined to a good stop-cock _d_, whic