22 CHEMICAL AND PHYSICAL EFFECTS. 



The interest, philosophical and practical, taken in 

 matters electrical at that time was intense, and far out- 

 weighed that taken in heat. The discovery of Oersted, in 

 1820, that the compass needle is deflected from its usual 

 direction by an electric current parallel to the needle, 

 followed by Sturgeon's discovery, in 1825, of the soft iron 

 electro-magnet, had, in 1837, rendered the electric telegraph 

 practical in the hands of Cooke and Wheatstone. Ohm 

 had, in 1827, discovered the relation between the electro- 

 motive force of the battery, the resistance of the circuit, 

 and the current generated. Faraday had discovered the 

 current caused in a closed circuit by the motion of a 

 magnet — " magneto electric induction " — and was engaged 

 in his now classical " Experimental Researches in 

 Electricity," having already shown that the quantity 

 of electricity produced in the battery is proportional 

 to the number of chemical equivalents electrolyzed. In 

 1836, Sturgeon added a new interest to the subject by his 

 invention of the " Commutator," and his construction by its 

 means of two machines, one the magneto-electric machine, 

 now called the " dynamo," and the other the electro-magnetic 

 engine, now called the " motor." By the former of these, a 

 current of electricity, having apparently all the properties of 

 a voltaic current, was produced by expending mechanical 

 effect in turning an electro-magnet between the poles of a 

 stationary magnet ; while by the second, a voltaic current 

 from a battery, acting on the coil of an electro-magnet 

 turning between the poles of a stationary magnet, urges it 

 to overcome resistance and produce mechanical effect. 

 These machines played an important part in the discovery 

 of the law of conservation of energy. By the first, an 

 electric current is produced solely by the expenditure of 



