Production of Statical Electricity by Heat and Light. 451 



to be measured to be introduced into the cage and placed on 

 a plate PP, which could be raised so as to bring the body 

 near the plate V connected to the electrometer. The 

 deflexion o£ the electrometer when P is raised will be 

 proportional to the charge of the body on it. 



To £l£CTf?Q METER 



As in fine weather the air is positively and the earth 

 negatively charged, I thought it desirable, as mentioned 

 above, to investigate what was the influence of sunshine on 

 the charge of bodies exposed to its action. For this purpose 

 plates o£ different substances were exposed in sunshine, and 

 their electrical charge before and after exposure measured. 



In making the experiments I observed that a piece of 

 paraffin wax and a sheet of gutta-percha which had lain in 

 the dark for at least a year, showed even before exposure 

 a negative charge ; a stick of sealing-wax which had lain 

 in the ordinary light of a room showed also a slight negative 

 charge ; while the charge on an old ebonite plate and on a 

 glass rod were scarcely perceptible. After exposure to sun- 

 light all these bodies were charged : the paraffin, gutta-percha, 

 and sealing-wax were highly charged, the charge on all 

 three being of negative sign. The ebonite plate showed a 

 slight negative, and the glass rod a strong positive charge. 



