118 NOTES TO BOOK XIII. 



place by conditions which the usual nature and form of 

 voltaic apparatus suggest, has been able to give great sim- 

 plicity to his reasonings. These conditions are, the linear 

 form of the conductors (wires) and the steadiness of the 

 electric state. For this part of the problem Dr. Ohm's 

 reasonings are as simple and as demonstrative as the ele- 

 mentary propositions of Slechamcs. The formulae for the 

 electric force of a voltaic current to which he is led have 

 been experimentally verified by others, especially Fechner 

 (Mass-bestimmengen iiber die Galvanische Kette. Leipzig, 

 1831,) Gauss, in the Results of the Magnetic Association ; 

 Lenz, Jacobi, Poggendorf, and Pouillet. 



Among ourselves, Mr. Wheatstone has confirmed and 

 applied the views of M. Ohm, in a Memoir On New Instru- 

 ments and Processes for determining the Constants of a Vol- 

 taic Circuit, (Phil. Trans. 1843. Pt. n.) He there remarks, 

 that the clear ideas of electromotive forces and resistances, 

 substituted by Ohm for the vague notions of quantity and 

 intensity which have long been prevalent, give satisfactory 

 explanations of the most important difficulties, and express 

 the laws of a vast number of phenomena in formulae of 

 remarkable simplicity and generality. In this Memoir, 

 Professor Wheatstone describes an instrument which he 

 terms the Rheostat, because it brings to a common standard 

 the voltaic currents which are compared by it. He gene^ 

 ralizes the language of the subject by employing the term 

 rheomotor for any apparatus which originates an electric 

 current (whether voltaic or thermoelectric, Sic.) and rheo^ 

 meter for any instrument to measure the force of such a 

 current. It appears that the idea of constructing an in- 

 strument of the nature of the Rheostat had occurred also 

 to Prof. Jacobi, of St. Petersburg. 



