1177.  As far as experiment has proceeded, it appears, therefore, impossible either to evolve or make disappear one electric force without equal and corresponding change in the other.  It is also equally impossible experimentally to charge a portion of matter with one electric force independently of the other.  Charge always implies induction, for it can in no instance be effected without; and also the presence of the two forms of power, equally at the moment of the development and afterwards.  There is no absolute charge of matter with one fluid; no latency of a single electricity.  This though a negative result is an exceedingly important one, being probably the consequence of a natural impossibility, which will become clear to us when we understand the true condition and theory of the electric power.

1178.  The preceding considerations already point to the following conclusions:  bodies cannot be charged absolutely, but only relatively, and by a principle which is the same with that of induction.  All charge is sustained by induction.  All phenomena of intensity include the principle of induction.  All excitation is dependent on or directly related to induction.  All currents involve previous intensity and therefore previous induction.  INDUCTION appears to be the essential function both the first development and the consequent phenomena of electricity.

P iii. Electrometer and inductive apparatus employed.

1179.  Leaving for a time the further consideration of the preceding facts until they can be collated with other results bearing directly on the great question of the nature of induction, I will now describe the apparatus I have had occasion to use; and in proportion to the importance of the principles sought to be established is the necessity of doing this so clearly, as to leave no doubt of the results behind.

1180. Electrometer.—­The measuring instrument I have employed has been the torsion balance electrometer of Coulomb, constructed, generally, according to his directions[A], but with certain variations and additions, which I will briefly describe.  The lower part was a glass cylinder eight inches in height and eight inches in diameter; the tube for the torsion thread was seventeen inches in length.  The torsion thread itself was not of metal, but glass, according to the excellent suggestion of the late Dr. Ritchie[B].  It was twenty inches in length, and of such tenuity that when the shell-lac lever and attached ball, &c. were connected with it, they made about ten vibrations in a minute.  It would bear torsion through four revolutions or 1440 deg., and yet, when released, return accurately to its position; probably it would have borne considerably more than this without injury.  The repelled ball was of pith, gilt, and was 0.3 of an inch in diameter.  The horizontal stem or lever supporting it was of shell-lac, according to Coulomb’s