CHAP, vii.] ELECTRICITY AND MAGNETISM. 337 



pencils of hard carbon separated by a thin layer of plaster 

 of Paris and supported in an upright holder. The arc 

 plays across the summit between the two carbon wicks. 

 In order that both carbons may consume at equal rates, 

 rapidly alternating currents must be employed, which is 

 disadvantageous from an economical point of view. 



374. Light by Incandescence. Voltaic arcs of 

 an illuminating power of less than 100 candles cannot 

 be maintained steady in practice, and 



are uneconomical. For small lights it 

 is both simpler and cheaper to employ 

 a thin continuous filament of some in- 

 fusible conductor, heated to whiteness 

 by passing a current through it. Thin 

 wires of platinum have repeatedly been 

 suggested for this purpose, but they 

 cannot be kept from risk of fusing. 

 Thin filaments of carbon have lately 

 been employed by Swan, Edison, and 

 others, for the construction of little in- 

 candescent lamps, the filament being pig. 140. 

 placed in a glass bulb exhausted to a 

 very high degree of air or other gases. In Swan's lamp 

 (Fig. 140) the carbon thread is no thicker than a horse- 

 hair, and of as much as i oo ohms resistance when cold : 

 the filament becomes remarkably elastic and metal-like 

 after being used. 



LESSON XXXIII. Electromotors {Electromagnetic 

 Engines). 



375. Electromotors. Electro-magnetic engines, or 

 electromotors, are machines in which the motive power 

 is derived from electric currents by means of electro- 

 magnets. In 1821 Faraday showed a simple case of 

 rotation produced between a magnet and a current of 

 electricity. In 1831 Henry, and in 1833 Ritchie, con- 



