338 



Lord Kelvin arid Messrs. Maclean and Gait. [Feb. 21 r 



air, if unelectrified* when entering it, must have been negatively- 

 electrified when leaving it. 



7. To test if the air was negatively electrified after bubbling, on 

 the llth of October the apparatus^ shown in fig. 4 was set up. The 



FIG. 4. 



apparatus consists of a large sheet iron vat, YV, 123 cm. in diameter 

 and 70 cm. in height, inverted on a large wooden tray lined with 

 lead, and supported by three blocks of wood. By filling the tray 

 with water, the air is confined in the vat. 



CO is a metal screen kept metallically connected with the case of the 

 electrometer, and with the vat. It surrounds both the electrometer 

 and the water-dropper, to prevent any external varying electrifica- 

 tions from vitiating the proper results of our experiments. 



This screening of the electrometer is absolutely necessary when it 

 is used with high sensibility (70 scale divisions per volt in our experi- 

 ments) in a [laboratory or other place where various other electric ex- 

 periments may simultaneously be going on. Four years ago the electro- 

 meter, the vat, and the water- dropper, were set up on the class-room 

 table without a metal screen. When the deflection indicated about 

 4 volts negative (see 8), the negative lead of Lord Kelvin's house 

 electric-light circuit, which passes through the class-room, was joined 

 to earth. This changed the deflection of the electrometer suddenly 

 by 1 volt* in the positive direction. When the positive lead was 

 " earthed," the deflection was changed suddenly by 6 volts in the 

 negative direction. Putting on sixteen 8 c.p. electric lamps, eight on 

 each side of the class-room, changed the deflection by two-thirds of 

 a volt in the negative direction. 



* Air was similarly blown from bellows into the vat (see 7) without any 

 bubbling, and no electrification was observed. 



f The vat, the water-dropper, and the electrometer are the same as in the 

 apparatus described in the ' Proceedings of the Koyal Society,' vol. 56, year 1894, 

 ' ' Electrification of Air," by Lord Kelvin and Magnus Maclean. 



