July 19, 1894] 



NA TURE 



281 



of the electrometer was 21 scale divhionj (half-millimetres) per 

 volt, and as the scale was 90 centimetres long, difference of 

 potentials up to 43 volts positive or negative, could be read by 

 adjusting the metallic zero to the middle of the scale. A fric- 

 tional plate-electric machine was used, and by means of it, in 

 connection with the pin-point, the air inside the vat could be 

 electrified either positively or ne;5atively. 



§ 7. The vat was fixed in position in the Apparatus Room of 

 the Natural Philosophy Department of the University of Glas- 

 gow on December 13, 1893, and for more than three months the 

 air inside was left undi-^turbed except by discharges from the 

 pin-point through the electrifying wire, and by the spray from 

 the water-dropper. Thus the air was becoming more and more 

 freed of dust day by day. Vet at the end of the four months we 

 found that the air was as easily electrified, either positively or 

 negatively, as it was at the beginning ; and that if we electrify 

 it strongly by turning the machine for half an hour, it retains a 

 considerable portion of this electrification for several hours. 



§ 8. Oiiservations were taken almost daily since December 13 ; 

 but the following, taken on February 8, March 12, and April 23, 

 will serve as specimens, the results being shown in each case by 

 a curve. At all these dates the air must have been very free 



the curve was taken one minute afterwards, or ten minutes after 

 the machine stopped turning (35'25 volts). 



Curfe ■},. .March 12, 1894 — A Voss induction machine was 

 joined to the charging wire, and run by an electric motor for 

 four hours nineteen minutes. A. test was applied at the be- 

 ginning of the run 10 make sure that it was charging negatively ; 

 and a .similar test when it was disconnected from the charging 

 wire in the vat showed it to be still charging negatively. The 

 water-dropper was joined to the electrometer, and the spot 

 appeared on the scale immediately. The first reading on the 

 curve was taken half a minute after the machine was disc m- 

 nected (30 65 volts). 



Ciine 4. April 23, 1S94. — The friction-plate machine was 

 turned positive for thirty seconds, with water-dropper running 

 and joined to the electrometer. Twenty seconds after the 

 machine stopped the spot appeared on the scale, and the read- 

 ing one and a half minutes after the machine stopped turning 

 is the first point on the curve (7'3 volts). 



Curve 5. April 23, 1894. — The friction-plate machine was 

 turned negative for thirty seconds, with the water-dropper run- 

 ning and joined to the electrometer. Ten seconds afterwards 

 the spot appeared on the scale, and the reading seventy seconds 



SSSI; 



TUBE Of- WATER- DROPPEfl 



yPARAFFIN 



'^^^'■f^ 



- METAL TUBC. 



Fig. 3. 



from dust. Both during the charging and during the observa- 

 tions the case of the electrometer and one pair of quadrants are 

 kept metallically connected to the vat. During the charging 

 the water-dropper and the other pair of quadrants were also kept 

 in connection with the vat. Immediately after the charging 

 was stopped the charming-wire was connected metallically to the 

 outside of the vat, and left so with its sharp point unchanged in 

 its position inside the v.at during all the ot)servations. 



§ 9. Cune I. February 8, 1894. — The friction-plate machine 

 was turned positive for half an hour. Ten minutes after the 

 machine stopped the water-dropper was filled and joined to 

 one pair of quadrants of the electrometer, while the other pair 

 was joined to the case of the instrument. The first reading on 

 the curve was taken four minutes afterwards, that is, fourteen 

 minutes after the machine stopped running (18 volts). 



Curve 2. March 3, 1894. — The friction-plate machine was 

 turned positive for five minutes. The water-dropper was filled 

 and joined to the electrometer immediately after the machine 

 stopped turning. The spot was off the scale, and nine minutes 

 elapsed before it appeared on the scale. The first reading on 



NO. 1290, VOL. 50] 



10 



15 20 25 50 35 



Ttmc in. min-uCes 



40 45 



after the nachine stopped turning is the first point on the cuive 

 (7-6 volts). 



The curves show, what we always found, that the air does 

 not retain a negative electrification so long as it retains a 

 positive. We also found, by giving equal numbers of turns to 

 the machine, that the immediately resulting difference of poten- 

 tial between the water-dropper and the vat was greater for the 

 negative than for the posiiive electrification ; though the 

 quantity received from the machine was probably less in the 

 case of the negative electiificition, because the negative con- 

 ductor was less well-insulated than the posiiive. 



§ 10. On March 21, two U-tubes were put in below the edge 

 of the vat, one on either side, so that it might be possible to 

 blow dusty, or smoky, or dustless air into the vat. To one tube 

 was fitted a blowpipe bellows, and by placing it on the top of a 

 box in which brown paper and rosin were burning, the vat was 

 filled with smoky air. Again, several layers of cotton-wool 

 were placed on the mouth 01 the bellows, so as to get dustless 

 air into the vat. The bellows were woiked for several hours 

 on four successive days, and we f jund no appreciable difference 

 (i) in the ease with which the air could be electrified by dis- 

 charges from the wire connected to the electric machine, and 

 (2) in the length of time the air retains its electrification. 



But it was found that, as had been observed four years ago 

 with the same apparatus,' with the water-dropper insulated and 

 connected to the electrometer, and no electrification of any 

 kind to begin with, a negative electrification amounting to four, 

 five, or six volts gradually supervened if the water-dropper was 



1 Maclean and Goto, Philosophical Magazine, .August 1S90. 



