THE HISTORY OF OHM'S LAW 



607 



depends upon tlie ratio of length to cross-section. The second experi- 

 ments comprised the comparison of the currents flowing tlirough two 

 conductors of equal length and equal cross-section, one of which was of 

 circular and the other of very flat section. The result showed that with 

 the same electromotive-force the currents were also the same, thus prov- 

 ing the current to be uniformly distributed throughout the section. 

 His third series of experiments gave a verification of the laws of 

 parallel resistances. These results constitute a full experimental proof 

 of part II. of Ohm's law, which, in its entirety, had not been given up to 

 this time. 



3. Experiments showing the relation between the magnetic effect of 

 the current, the electromotive-force of the cell, and the length of wire 

 in the circuit. This is the relation which we have denominated part I. 

 of Ohm's law. He first presented it in the rather unfamiliar form of 

 the following equation. 



X = a/(h+x), (4) 



OfSion 



O^mS Toi^SioV? BftlAnCe ft»ic( Thermocouple 



Fig. 3 



where X is the magnetic effect of the current, a and & are constants 

 depending respectively upon the electromotive-force and the internal 

 resistance of the source of current; x is the length of wire constituting 

 the external resistance under test. In modern language equation (4) 

 may be written, 



