402 Mr. S. H. Freeman on the Question 



In order to compare the deflections obtained by evaporation 

 with those obtained by friction, the experiments of Tait and 

 Wanklyn were repeated. It was impossible, however, to 

 measure the deflections obtained, since in almost every case, 

 as soon as the spheroidal state ceased, the spot of light went 

 beyond the limits of the scale. 



To show the sensitiveness of the apparatus to very slight 

 friction, it was so arranged that drops of liquid could fall upon 

 the evaporating-dish from an insulated conductor in metallic 

 connexion with it. A few drops of liquid falling three or four 

 centimetres were sufficient to give deflections of nearly the 

 size obtained from a Daniell's cell, and much larger than any 

 obtained in evaporation. 



In order to compare the results obtained with the pheno- 

 mena of atmospheric electricity, the writer computed the 

 virtual depth of water which would be necessary to produce 

 one flash of lightning if the vapour in a thunder-cloud had 

 been electrified as indicated by the experiments. By virtual 

 depth is meant the depth of the layer of water over the whole 

 area of the cloud, were all its vapour condensed to the liquid 

 state. 



Let A = this depth in centimetres for a cloud of area A 

 and height a, 

 V and C = respectively the potential and capacity of 



the cloud, 

 q = the charge per gramme of water ; 

 then 



But, from Sir Win. Thomson's experiments, V = 130a*; 

 and by the ordinary formula for a condenser of horizontal 

 plates, 



vna 

 Now, in the experiments, 



let d — the deflection obtained by insulating for t minutes, 



and apparently due to evaporation, 

 D = the deflection obtained from a Daniell's cell, 

 W= weight, in grammes, of liquid evaporated in T 



minutes, 

 v and c = potential and capacity of evaporating-dish and 



its connexions ; 



* Papers on Electricity and Magnetism, p. 259. 



