PHYSICAL SCIENCE 43 



The most far-reaching development in physical 

 science during the nineteenth century was in the 

 domain of electricity. When the century opened, 

 the Italian Volta (1745-1827) had only just discovered 

 the pile or cell which enabled experimenters to ob- 

 tain a steady electric current instead of the isolated 

 charges alone available before. 



It was soon found that these electric currents pro- 

 duced magnetic forces, and, by the invention of 

 galvanometers, instruments in which these forces were 

 used to deflect magnetised needles, the currents were 

 brought within the power of measurement, and the 

 electric telegraph made possible. The complemen- 

 tary discovery of Faraday (1791-1867), that the 

 movement of a magnet would produce an electric 

 current, barely perceptible to his apparatus, gave rise 

 to the astonishing development of electro-magnetic 

 machinery of the second half of the century. 



Faraday's researches were largely inspired by his 

 repugnance to the idea of action at a distance, and 

 his consequent continual search into the properties 

 of the insulating medium through which electric 

 forces must exert themselves. Clerk-Maxwell (1831- 

 1879) interpreted and extended Faraday's ideas in 

 mathematical form, and showed that the properties 

 of the medium, which were necessary to explain the 

 electrical effects, were identical with those already 

 required to explain the propagation of light. 



Hence it became possible to conceive of light as 



