PHYSICS OF NINETEENTH CENT. ELECTRICITY. 593 



had been instituted, the "hydro-electric" machine was constructed, as 

 shown in Fig. 302. A strong iron boiler with an internal fire was 

 mounted on four glass pillars. The steam was allowed to escape under 

 a pressure of several atmospheres by a number of jets (a) of a peculiar 

 construction. The steam was directed against a rod v connected with 

 the insulated conductor B. The steam was positively and the boiler 

 negatively electrified. A machine of this kind used to be exhibited 



FIG. 304. 



at the Polytechnic Institution in London, where the boiler was 6| feet 

 long, and there were forty-six steam-jets. Sparks 22 inches in length 

 could be drawn from the conductor. The electricity of the Armstrong 

 machine is generated by the friction of particles of water against the 

 sides of the wooden orifices from which the steam issues. Dry steam 

 is found to produce no electricity: hence the tubes by which the steam 

 escapes include an arrangement for obtaining a slight previous con- 

 densation of the steam. 



More recently some machines have been invented which supply 

 electricity of high tension, that is, electricity like that of the common 

 factional machine, merely by the motion of a surface under induction. 

 Figs. 303 and 304 represent one of the most effective of these machines, 



38 



