882 Boyal Society. 



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 the 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 rheomoter 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, whatever may be the number 

 of elements of which the battery consists. The electromotive forces 

 of voltaic elements formed of an amalgam of potassium with zinc, 

 copper and platina, a solution of a salt of the negative metal being 

 the interposed liquid, are given ; the last combination is one of great 

 electromotive energy, and when a voltameter is interposed in the 

 circuit, it decomposes abundantly the water contained in it. A still 

 more energetic electromotive force is exhibited by a voltaic element, 

 consisting of amalgam of potassium, sulphuric acid, and peroxide of 

 lead. The author then shows, that if three metals be taken in their 

 electromotive order, the electromotive force of a voltaic combination 

 formed of the two extreme metals is equal to the sum of the electro- 

 motive forces of the two elements formed of the adjacent metals. 



Among the instruments and processes described in the subsequent 

 part of the memoir are the following. 1. An instrument for mea- 

 suring the resistance of liquids, by which the errors in all previous 

 experiments are eliminated, particularly those resulting from neglect- 

 ing the contrary electromotive force arising from the decomposition 

 of the liquid. 2. The differential resistance measurer, by means of 

 which the resistances of bodies may be measured in the most accu- 



