220 LORD KELVIN, DR. M. MACLEAN, AND MR. A. GALT, ON THE 



two paraffin blocks. The filter was of block-tin tube, 5 centims. long and 0'6 

 centim. bore, and was filled with fine brass filings. It was connected by a short 

 platinum wire to the insulated terminal of a quadrant electrometer, F, and beyond 

 the filter were tubes passing to a mercury gauge, G, and air-pump, H. The can was 

 connected to the uninsulated terminal of the electric machine and to the sheaths of 

 the voltmeter and electrometer. 



74. The experiment was conducted as follows : Apertures Nos. 1, 2, 3, 4, and 7 

 in the can were closed, and the electric machine started, the air in the can being 

 charged by a brush discharge from the needle point. The electrometer terminal 

 joined to the filter was insulated, and the pump worked for some time, fresh air 

 filtered through cotton-wool entering the can by a pipe attached to aperture No. 6. 

 The tests were made at potentials ranging from 2500 up to 12,000 volts, and it 

 was found that 3200 volts negative and 4000 volts positive gave about the best 

 results. The speed of the pump was kept constant, and the cubic contents of the 

 cylinders of the pump and the electric capacity of the insulated filter and quadrants 

 of the electrometer being known, the quantity of electricity in absolute measure 

 taken from each cub. centim. of air by the filter could be determined. In experi- 

 menting with carbonic acid gas, the procedure adopted was almost exactly the same 

 as that for air, the only difference being that instead of admitting air by the pipe 

 attached to aperture No. 6 the same pipe was attached to the nozzle of an upright 

 pressure cylinder containing carbonic acid gas. The gas was admitted to the can 

 under very slight pressure. For carbonic acid gas, the charging potentials which 

 gave the best results were found to be about 4000 volts negative and 5000 volts 

 positive. 



In order to find out the electric density of the electrified air or carbonic acid 

 gas when left in the can for some time after charging had ceased, the electrification 

 was stopped after the machine and pump had been worked for several minutes. 

 The charging wire was removed from the needle and the apertures in the can 

 blocked. The enclosed electrified air or carbonic acid gas was left to itself for 

 different times in different experiments, generally just l hours. The gas in the 

 can was then drawn from No. 5 aperture through the insulated filter to the pump 

 (aperture No. 6 being opened to admit fresh air), and the pump was stopped when 

 all signs of electrification ceased. 



The following results were obtained : 



