Page:Proceedings of the Royal Society of London Vol 4.djvu/496

Rh a short account of the principal results to which this theory leads,

and shows how the clear ideas of electromotive forces and resist- ances, substituted for the vague notions of intensity and quantity which formerly prevailed, furnish us with satisfactory explanations of phenomena, the laws of which have hitherto been involved in ob- scurity and doubt. According to Ohm's system, the force of the current is equal to the sum of the electromotive forces divided by the sum of the resistances in the circuit. The several electromotive forces and resistances which enter into the circuit of a voltaic battery are then defined ; and having frequent occasion to refer to the laws of the distribution of the electric current in the various parts of a circuit, when a branch conductor is placed so as to divert a portion of the current from a limited extent of that circuit, the author directs particular attention to these laws. After recommending several new terms in order to express general propositions, without circum- locution and with greater precision, the author states the method of obtaining the constants of a circuit employed by Fechner, Lenz, Pouillet, &c., and then ]iroceeds to explain the nev/ method he has himself adopted. The principle of this method is the employment of variable instead of constant resistances, bringing, thereby, the currents in the circuits compared to equality, and inferring from the amount of the resistance measured out between two deviations of the needle, the electromotive forces and resistances of the circuit according to the particular conditions of tlie experiment ; a method which requires no knowledge of the forces corresponding to differ- ent deviations of the needle. To apply this principle, it is requisite to have a means of varying the interposed resistance, so that it may be gradually changed within any required limits. The author de- scribes two instruments for effecting this purpose ; one intended for circuits in which the resistance is considerable, the other for circuits in which it is small. The Rheostat (for thus the inventor names the instrument under both its forms) may also be usefully employed as a regulator of a voltaic current, in order to maintain for any required length of time precisely the same degree of force, or to change it in any required proportion ; its advantages in regulating electro-mag- netic engines and in the operations of voltatyping, electro-gilding, &c. are pointed out.

Various methods of measuring the separate resistances in the cir- cuit, particularly that of the rlieomoter itself, are next described ; and it is shown that the number of turns of the rheostat requisite to reduce the needle of a galvanometer from one given degree to another, is an accurate measure of the electromotive force of the circuit. It is then proved that similar voltaic elements of various mag- nitudes, conformably to theory, have the same electromotive force ; that the electromotive force increases exactly in the same propor- tion as the number of similar elements arranged in series ; and that when an apparatus for decomposing water is placed in a circuit, an electromotive force, opposed to that of the battery, is called into action, which is constant in its amount, Avhatever may be the number of elements of which the battery consists. The electromotive forces