172 SCIENTIFIC APPARATUS. 



B. When the strength of a current varies, or when the position of 

 the circuit relatively to other conductors or to magnets is changed, 

 more or less work may be done in the production of effects ex- 

 ternal to the circuit in addition to some internal work of one or 

 more of the above kinds. The external work may be of any of 

 the following kinds : 



(4) Magnetic induction; that is, the development of the 



magnetic condition in substances susceptible of mag- 

 netism. 



(5) Production of induced currents in other conductors. 



(6) Production of mechanical work through the motion of 



magnets or of conductors conveying currents. 



The laws according to which the action (i) takes place were 

 first ascertained by Joule (1841). The effect referred to under 

 (2) was discovered by Peltier (1834), and has since been 

 investigated chiefly by Edlund (1870 71). The laws of the 

 chemical action of the current (3) were established by Faraday 

 (1833). The magnetising power of the current (4) was first 

 observed by Arago (1820) ; and the production of induced currents 

 (5) by Faraday (1831). The existence of mechanical force between 

 electric currents and magnets, capable of doing work by changing 

 their relative position, was discovered by Oersted in 1820; and 

 in the same year Ampere discovered that there is a mechanical 

 force between two currents or two parts of the same current. 

 From the laws according to which he found that this force is 

 exerted, he deduced the conclusion that a closed electric circuit 

 has the properties of a magnet, and showed that any magnet 

 might be replaced by a system of circulating currents. 



Most of the effects produced by an electric current are con- 

 nected with the direction of the current in such a way that they 

 are inverted when the direction of the current is inverted. Thus, 

 when a current passes from a piece of bismuth into a piece of 

 antimony, heat is absorbed; but when it passes from antimony to 



