II.— ON THE ELECTRIFICATION OF THE ATMOSPHERE 

 SURROUNDING SOLID BODIES WHEN THESE 

 ARE RAISED TO MODERATE TEMPERATURES. 



By J. C. Beattie, D.Sc, F.R.S.E. 



§1. The effect of increase of temperature of the charged body on 

 its capabihty of retaining a charge has been made the subject of 

 research by numerous experimenters. 



Guthrie showed that a metal — in his experiments an iron sphere 

 — when wiiite hot cannot retain a charge either of positive or of 

 negative electricity ; and that as it cools it acquires the power of 

 retaining a negative charge before it can retain a positive one. 



Elster and Geitel have shown that a conductor insulated in the 

 neighbourhood of a glowing body becomes charged in air which pre- 

 viously had been rendered free from dust particles. These experi- 

 menters have also shown that a body made to glow in a gaseous 

 atmosphere takes a charge whose value depends on the nature and 

 ^tate of glow of the body as well as on the nature and density of the 

 surrounding gas. 



Schuster states that a glowing copper rod gives off negative 

 electricity to the air so long as it is oxidising. After the oxidation 

 is complete it retains a charge given it, whether the charge be 

 positive or negative. On the other hand, when an oxidised copper 

 wire was made to glow in hydrogen, it retained a negative but not a 

 positive charge so long as there was deoxidation, and even for a time 

 after the deoxidation had ceased. 



Branly states that the electrification of the air surrounding a 

 glowing surface depends on the nature of the surface. He found, 

 for an example, that a red-hot glass rod, when connected to the 

 ground, discharges negative electricity, while a lamp cylinder covered 

 with aluminium oxide, bismuth oxide, or lead oxide, discharges 

 positive electricity. 



J. J. Thomson has observed that a charged metallic body sur- 

 rounded by a heated atmosphere has greater power of communicating 

 a charge to the atmosphere when it is hot than it has when cold. 



Closely connected with the various phenomena mentioned 

 above is the well-known discharging power of flames with or without 

 volatilized substances in them. 



Of recent years it has been possible — thanks to Kelvin's electric 

 filter method — to examine not only the change in the electric state 

 of the solid, but also the change in that of the surrounding gas. 



