•i-i A. M. Mayer — Experimental Proof of Olimh Law. 



and Poisson, that even if there existed no other reasons, we 

 might with perfect justice draw the conclusion that there 

 exists an intimate connection between both natural phenomena ; 

 and this relation of identity increases, the further we pursue 

 it. These researches belong to the most difficult in mathemat- 

 ics, and on that account can only gradually obtain general 

 admission ; it is therefore a fortunate chance, that in a not 

 unimportant part of the propagation of electricity, in conse- 

 quence of its peculiar nature, those difficulties almost entirely 

 disappear." 



From these premises, and guided by results of experiments 

 made by him and by Ritter, Erman, Jager, Davy and Bee- 

 querel he arrived at the following conditions as existing in a 

 voltaic circuit. 



1. In a homogeneous conductor, forming part of a voltaic 

 circuit, the difference of the electric tensions at any two points 

 of the conductor is proportional to their distance. 



2. In different conductors forming part of a circuit, the 

 difference of tensions at two points separated by an interval 

 equal to the unit of length is in the inverse ratio of the sec- 

 tion of the conductor and of its coefficient of conductivity. 

 Hence, in different conductors, equal differences of tension 

 correspond to lengths whose electric resistance is the same. 



3. At the point of contact of two different conductors, there 

 is a sudden variation of electric tension. 



4. If A equals the sum of the electro-motive forces, L the 

 resistances, X the resistance reckoned from a point m of the 

 circuit to a point p when the tension is zero, the tension at 

 the point m is given by the formula 



u=A z . 



Ohm eventually arrives at the formula S=y, which expresses 



what is generally known as his law. Which formula, he says, 

 " is generally true, and already reveals the equality of the force 

 of the current at all points of the circuit ; in other words it 

 may be thus expressed : The force of the current in a galvanic 

 circuit is directly as the sum of all the tensions, and inversely 

 as the entire reduced length of the circuit, bearing in mind 

 that at present by reduced length is understood the sum of all 

 the quotients obtained by dividing the actual lengths corres- 

 ponding to the homogeneous parts by the product of the 

 corresponding conductivities and sections." 



The words " tension " (Spannung) and " electromotive force" 

 used by Ohm are the equivalent of the word potential. He 

 was the first to introduce this conception into the theory of 



