92 Galvanism, from Galvani to Ohm. 





or i [ds'. B], 



where B = 



Now this value of B is precisely the value found by Biot and 

 Savart* for the" magnetic intensity at ds' due to the- current i in 

 the circuit s. Thus we see that the ponderomotive force on a 

 current-element ds' in a magnetic field B is i' [ds'. B]. 



Ampere developed to a considerable extent the theory 

 of the equivalence of magnets with circuits carrying currents; 

 and showed that an electric current is equivalent, in its 

 magnetic effects, to a distribution of magnetism on any 

 surface terminated by the circuit, the axes of the magnetic 

 molecules being everywhere normal to this surface :f such a 

 magnetized surface is called a mayiwtic shell. He preferred, 

 I however, to regard the current rather than the magnetic fluid 

 as the fundamental entity, and considered magnetism to be 

 really an electrical phenomenon : each magnetic molecule owes 

 its properties, according to this view, to the presence within it 

 / of a small closed circuit in which an electric current is 

 perpetually flowing. 



The impression produced by Ampere's memoir was great 

 and lasting. Writing half a century afterwards, Maxwell 

 speaks of it as " one of the most brilliant achievements in 

 science." " The whole," he says, " theory and experiment, 

 seems as if it had leaped, full-grown and full-armed, from the 

 brain of the ' Newton of electricity/ It is perfect in form and 

 unassailable in accuracy ; and it is summed up in a formula 

 from which all the phenomena may be deduced, and which 

 must always remain the cardinal formula of electrodynamics." 



Not long after the discovery by Oersted of the connexion 

 between galvanism and magnetism, a connexion was discovered 

 between galvanism and heat.| In 1822 Thomas Johann Seebeck 



* See ante, p. 86. t Loc. cit., p. 367. 



