THE PROGRESS OF PHYSICS. 159 



the disturbance of the magnetic needle by the influ- 

 ence of an adjacent electrical current, he not only 

 made a step of great theoretical import, but pointed 

 forward (as we now recognise) to the invention of 

 the telegraph. 



Oersted's experiment suggested the possibility of 

 measuring the strength of an electric current by its 

 effect upon an adjacent magnet, and this led Schweig- 

 ger in 1820 to his invention of the galvanometer or 

 electrometer, a fundamental instrument in electrical 

 science. As the history of galvanometers alone is 

 a long one, we must be content here to note that after 

 modifications by Nobili and Pouillet and others, 

 the measuring instrument was brought to great per- 

 fection by Sir William Thomson (Lord Kelvin). 



Oersted observed the influence of a current on a 

 magnet, and that the latter always tends to set itself 

 at right angles to the direction of the current, but a 

 further step was soon taken by Ampere (1775-1836), 

 who showed (1820) that one current influences an- 

 other, parallel currents in the same direction being 

 attracted, those in opposite directions being repelled 

 by each other. His mathematical theory of these 

 phenomena is still referred to as a masterpiece. 



Ohm. To Ohm (1789-1854) the science was 

 greatly indebted for the precision which he gave to 

 the conceptions of electro-motive force, strength of 

 current, electric resistance and conductivity, and for 

 the law (experimentally established in 1826, mathe- 

 matically worked out in 1827) which states that 

 the resistance of a conductor can be measured by the 

 ratio of the electro-motive force between its two ends 

 to the current flowing through it. It appears that 

 this empirical generalisation had been reached in 

 1781 by Cavendish, but practically its recognition 



