238 GREAT MEN OF SCIENCE 



the magnetic needle as soon as temperature differences 

 exist in it. Seebeck called this 'magnetic polarisation of 

 metals by temperature differences' or 'thermo-magnetism.' 

 Ohm immediately regarded the discovery as the finding of a 

 new source of current, which entirely consisted of con- 

 ductors of the first class. We may also say that what had 

 been found was really that Volta's series changes its order 

 when the temperature is different. When Ohm introduced 

 a piece of bismuth into a circuit otherwise consisting of 

 copper, and kept one of the two points of contact in boiling 

 water and the other in ice, he obtained very constant cur- 

 rents, which he was able to observe by means of a very 

 simple galvanometer included in the circuit. 



This provided him with a trustworthy and invariable 

 source of electromotive force, and it thus became possible to 

 study the influence upon the current strength of the re- 

 sistances introduced into the circuit, in a trustworthy and 

 unobjectionable manner. It was thus rendered certain that 

 the resistance of a conductor in Ohm's law is proportional to 

 its length, and inversely proportional to its cross-section, and 

 to its conductivity. The connection with the cross-section 

 shows that electricity in constant flow, as contrasted with 

 that at rest, is not situated on the surface of a conductor, but 

 distributed throughout its whole interior. Ohm already 

 begins to consider the state of transition from rest to flow of 

 electricity, when the circuit is closed, although the knowledge 

 at that time did not allow him to deal with it satisfactorily. 

 But he already found quite correctly that the conductivity of 

 liquids, as opposed to that of metals, increases with increase 

 of temperature. 



Ohm came from an old Erlangen family. He received 

 from his father who was a master-mechanic, a careful edu- 

 cation; his mother died when he was young. During his 

 time at school he was also introduced successfully by his 

 father to mathematics and physics, who for this purpose, as 



