March 21, 1895] 



NA TURE 



407 



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

 the electrometer and the water-dropper, to prevent any external 



varying electrifications from 

 vitiating the proper results of 

 our experiments. 



This screening of the elec- ' 

 trometer is absolutely neces- 

 sary when it is used with high 

 sensibility (70 scale divisions 

 per volt in our experimenis) 

 in a laboratory or other place 

 where various other electric > 

 experiments may simultane- ^ 

 ously be goingon. Four years Jf 

 ago the electrometer, the vat, ° 

 and the water-dropper, were ■ 

 set up on the class-room table ^ 

 "vithotit a metal screen. When ^ 

 the deflection indicated about 

 4 volts negative {see § S), the 2 

 negative lead of Lord Kelvin's 

 house electric-light circuit, 

 which passes through the 

 class-room, was joined to 

 earth. This changed the de- 3 

 flection of the electrometer 

 suddenly by I volt in the posi- 

 tive direction. When the 

 positive lead was "earthed," 

 the deflection was changed 4 

 suddenly by 6 volts in the 

 negative direction. Putting 

 on sixteen 8 e.p. electric 

 lamps, eight on each side of 

 the class-room, changed the 5 

 deflection by two-thirds of a 

 volt in the negative direction. 

 § 8. In experimenting with 

 the same apparatus ' in 1890, it 



1 Phil. Mag. August, 1890, " 

 *' Electrification of Air by a Water 

 Jet," by ^Iaclean and Goto. 



NO. 1325, VOL. 51] 



CuRvas 6 AXD 7. 



was found that the water jet gave negative electricity to the 

 ordinary air of the laboratory enclosed in the vat. The 

 present experimenis fully confirm this 

 result, showing a gradual negative electri- 

 fication of the enclosed mass of air rising 

 to about S volts in an hour, once every 

 day for the first few days. For twenty- 

 eight days after the vat was set up, in 

 October 1894, fifteen observations of an 

 hour each were taken to find the effect of 

 the water-dropper, with no other disturb- 

 ing influence on the unchanged volume of 

 air inside the vat. These experiments 

 verify the conclusion (Phil. Ma^., August 

 1890) that the more the air inside the vat 

 became free of dust, the less became the 

 rate at which the air was negatively elec- 

 trified by the water-dropper. 



§ 9. On October 15 last the vat was lifted 

 from the tray to remove some obstruction 

 in the nozzle of the water-dropper, which 

 was not then flowing freely. Curve (6) was 

 obtained that afternoon. The air in the 

 vat was the ordinary air of the laboratory, 

 and the curve shows the effect of the 

 water-dropper alone in electrifying the air 

 negatively. For the next two days the 

 water dropper was kept running continu- 

 ously for about eight hours each day, to 

 wash the dust out of the air, and on October 

 18 curve (7) was obtained. It shows a much 

 less rate of negative electrification than 

 curve (6). In the experiments of summer 

 1890 an aspirator was used to draw the air 

 from the vat, and a tube full of cotton-wool 

 was used to filter the air drawn into 

 the vat. 



Curves (i) to (5) are reproduced from 

 the Philosophical Magazine, and they 

 show that the more the air becomes free 

 from dust the less is the rate at which the 



CORrES I TO 5. 



