GEORG SIMON OHM 237 



electromotive force in the circuit and also that of current 

 strength, and he was also able to state his result in a definite 

 manner, in the form of equations, the most important by far 

 being Ohm's law: 'The current is equal to the driving tension 

 or electromotive force divided by the resistance.' If this law 

 is applied not only to the whole circuit, but also to any part of 

 it, the distribution of electromotive force can be found in any 

 part of the circuit. 



A part of his experiments were performed with voltaic 

 elements; in particular, he thus demonstrated the fall in 

 potential along a long and thin connecting wire, for which 

 purpose he, like Volta, made use of the condenser. In this 

 way it could be proved - a question left open by Ampere, - 

 that the voltage of the force of current does not disappear 

 when the circuit is closed, but that it distributes itself in a 

 definite way, in accordance with the resistances. Ohm 

 also recognised that the sources of current discovered by 

 Volta possess the property of maintaining a definite difference 

 of tension at the points of contact of the different conductors, 

 whether the circuit is open or closed, this difference in 

 tension (electric potential) depending only on the nature of 

 the conductors concerned. This difference in tension is also 

 called the electromotive force of the current source in ques- 

 tion. The fact that this is not very constant in an ordinary 

 voltaic element, but fluctuates as Ohm says, depends, as 

 Ohm already saw, upon the chemical changes which take 

 place of themselves in the element when the current is used. 



The constant galvanic element, which avoids these dis- 

 turbing chemical changes, had not then been discovered, 

 and this was at first a serious difficulty for Ohm. But he 

 found a means of getting over this, by following Poggen- 

 dorf's advice to use the thermo-electric elements just dis- 

 covered as a source of current. This was a discovery made 

 in the year 1821 by Seebeck (lived between 1770 and 1831 in 

 Germany), that a circuit formed of different metals deflects 



