222 THE DEVELOPMENT OF ELECTETCAL SCIENCE. 



things, Faraday set himself to explain this. The result was the dis- 

 covery of the commutatorless dynamo, or Faraday disk. In view of 

 modern developments, probably the most important of Faraday's dis- 

 coveries was that of the production of a current in a circuit when a 

 current is either established or varied in strength in an adjacent cir- 

 cuit. This was followed by the discovery that relative motion of two 

 circuits, one of which carried a current, produced a current in the other, 

 and that the motion of a magnet in the neighborhood of a circuit pro- 

 duced a current in the circuit. Another important discovery by Fara- 

 day was that of the quantitative laws which govern electrolytic decom- 

 position, thus giving us our electro-chemical equivalents. 



At this time Lenz was led by experiment to the discovery of his 

 celebrated law of induction, namely, that the current produced always 

 in turn produces forces tending to oppose the change. For example, 

 if a current be induced in a coil by bringing a magnet toward it the 

 mutual action between the magnet and the current is to oppose the 

 magnet's approach. This is important when looked at from the point 

 of view of the conservation of energy or as an argument against per- 

 petual motion. Lenz's law is, of course, when the actions are properly 

 understood, a consequence of Newton's third law of motion. 



Discoveries similar to those of Faraday as to induced currents were 

 made almost simultaneously by Henry in this country. We have in 

 the discoveries of Faraday and Henry the fundamental information 

 required for nearly the whole of our recent developments in dynamo- 

 electric generators and electric motors, but it was reserved for the next 

 generation to develop them. This development we owe in no small 

 degree to the splendid exposition of Faraday's discoveries and their 

 consequences contained in Maxwell's book on electricity and magnetism. 



Going back for a moment to 1822, we have to notice another important 

 discovery, namely, the thermoelectric couple by Seebeck. There fol- 

 lowed almost immediately the important experiment of Gumming, who 

 showed that the thermoelectric order of the metals is not the same at 

 all temperatures. 



The next important discovery in thermoelectricity was that of Peltier, 

 of the heat generated at the junction of two metals when a current is 

 forced across it against the electro-motive force of the junction. In 

 later years we have the classic researches of Thomson (Kelvin), who 

 added thermoelectric convection and the specific heat of electricity 

 and gave the thermoelectric diagram method of representing results. 

 This method was afterwards used and extended by Tait, who added a 

 good deal to our knowledge of thermoelectric data. Among the large 

 number of others who have worked in this field we may mention 

 Becquerel, Magnus, Matthieson, Leroux, and Avenarius. Thermo- 

 electric batteries of considerable power have been made by Glamond 

 and others. 



In 1827 the celebrated law giving the relation between electro-motive 



