AKD MODERN puvsica 157 



i 



behave as though its mass wore negative it would 

 move in a direction opposite to that of the force.* 



Since 1840 many other theories have been pro- 

 posed to explain Ampere's laws. Meanwhile, in 1821, 

 Faraday observed that under certain circumstances a 

 wire carrying a current could bo kept in continuous 

 rotation in u magnetic field by the action between the 

 magnets and the current. In 1824 Arago observed 

 the motion of a magnet caused by rotating a copper 

 disc in its neighbourhood, while in 1831 Faraday 

 began his experimental researches into electro-magnetic 

 induction. About the same period Joseph Henry, of 

 Washington, was making, independently of Faraday, 

 experiments of fundamental importance on electro- 

 magnetic induction, but sufficient attention was not 

 called to his work until comparatively recent years. 



In 1833 Lenz made some important researches, 

 which led him to discover the connection between the 

 direction of the induced currents and Ampere's laws, 

 summed up in his rule that the direction of the 

 induced current is always such as to oppose by its 

 electro-magnetic action the motion which induces it. 



In 1845 F. E. Neumann developed from this law 

 the mathematical theory of electro-magnetic induction, 

 and about the same time \V. Weber showed how it 

 might be deduced from his elementary law of 

 electrical action. 



The great name of Von Helmholtz first appears in 

 connection with this subject in 1851, but of his 

 writings we shall have more to say at a later stage. 



J. J. Thomson, B.A., Report, 1885, pp. 109, 113, Report on 

 it al Theories. 



