1185.  Although the constant angular distance of 30 deg. between the centres of the balls was adopted, and found abundantly sensible, for all ordinary purposes, yet the facility of rendering the instrument far more sensible by diminishing this distance was 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, which is itself attached either to the metallic foot e, or to an air-pump.  The aperture within the hemisphere at f is very small:  g is a brass collar fitted to the upper hemisphere, through which the shell-lac support of the inner ball