108 On the Electrification of Air by Uranium. 



experiments with voltages less than 500 or 600 volts per cm. 

 of line of force in the air at ordinary atmospheric pressure, 

 seem to be not sensibly influenced by disruptive charges or 

 by brushes ; and the quasi-conductivity of air produced by 

 uranium was the dominant factor. This is undoubtedly the 

 case in the experiments described by Dr. Beattie, and I assume 

 it to be so in what follows except when I give express 

 warning of possible liability to disruptive discharges. 



The effective conductivity produced in the air by the 

 uranium influence is, of course, greatest in the immediate 

 neighbourhood of the uranium, but there is something of it 

 throughout the enclosure. Hence it may be expected that 

 electricity of the same kind as that of the uranium will be 

 deposited in the air close around it, and electricity of the 

 opposite kind in the air near the enclosing metal surface. 

 According to our former experiments, the quantity flowing 

 from either the uranium or from the surrounding metal per 

 sq. cm. of its surface increases but little with increased 

 voltage when this exceeds 5 or 10 volts per cm. Now, when 

 the greatest diameter of the uranium is small in comparison 

 with distances to the outer metal surface, the voltage per 

 cm. is much greater along the lines of force near the uranium 

 surface than near their outer ends on the surrounding metal. 

 Hence the rate of discharge of electricity into the air from the 

 uranium will cease to increase sensibly with increasing differ- 

 ence of potential between the uranium and the surrounding 

 metal ; while the rate of discharge of the opposite electricity 

 from the large surrounding metal surface is still notably in- 

 creasing. Hence if the dimensions and shapes of the uranium 

 and of the surrounding metallic surface are such that for small 

 voltages, such as 10 or 20 volts of difference between the 

 uranium and the surrounding metal, the electricity lodged in 

 the air by discharge from the uranium preponderates over 

 that discharged from the surrounding metal, the excess 

 come to a maximum and diminish, possibly even down 

 to zero, with greater differences of potential. At potential- 

 differences still greater the electricity lodged in the air from 

 the outer metal may preponderate, and the electricity in the 

 air drawn off, and given to the filter, be of opposite sign to 

 that of the uranium which was found with the lower voltages: 

 provided the configurations are such, and the voltages are so 

 moderate, that disruptive discharge does not intervene to any 

 practically disturbing extent. 



