J56 SCIENTIFIC APPARATUS. 



machines in the ordinary sense. The chief purpose for which 

 they were proposed was to develope an easily recognisable degree 

 of electrification from a charge which was too slight to be directly 

 detected. Employed in this way, however, they were found 

 frequently to give contradictory results. They consequently fell 

 into disuse, and the principle of their construction seems almost 

 to have been lost sight of, until it was revived within recent years 

 in the construction of actual electrical machines by Varley (1860), 

 Holtz (1865), Topler (1865), and Sir William Thomson (1867, 

 1868). In the various forms of electrical machines constructed by 

 Holtz, the part which takes the place of the body B, hitherto 

 spoken of as a conductor, is a revolving glass plate ; consequently, 

 the electrical interchange between it and the parts which represent 

 A and c can only affect the portions of it which successively come 

 into actual communication with them. 



In all machines of the class here referred to, as well as in the 

 electrophorus, a limited initial charge of one kind is sufficient to 

 develope an indefinite amount of positive and negative electricity. 

 It is to be observed, however, that this is obtained at the ex- 

 pense of mechanical energy employed in maintaining the motion 

 of the machine in opposition to electrical attraction and repulsion, 

 according to the well-known law that there is an increase of 

 electrical energy when work is done against electrical forces. 



Besides the methods indicated above for developing the elec- 

 trical condition, many other processes are known whereby a 

 similar result can be produced. Of these, the contact of hetero- 

 geneous conductors, in conjunction with chemical action, as in 

 the voltaic battery (Volta, 1800), or in conjunction with differ- 

 ence of temperature, as in the thermo-electric battery (Seebeck, 

 1823), and the movement of electric conductors in a magnetic 

 field, or the variation of the strength of a magnetic field in 

 which there are stationary conductors (Faraday, 1831), are those 

 which have hitherto been of the greatest practical importance. 

 Each of these principles has been applied, with very numerous 



