Ji(jCLEi,LANU AND Noi.AN — Tlic Electvic Charge on Rain. 85 



of the observer's time, the "results obtaiued have led us to continue the work 

 in the same manner. 



During the observations of the charge on raiu, the sign and variations of 

 potential at a point in the air at some distance from the apparatus foi- 

 collecting the raiu were also observed. An insulated wire was connected 

 to a suitable electroscope near the observer, and a sheet of lead-foil possessing 

 induced activity was wrapped around the wire at the point where the potential 

 was required. ■ The electroscope consisted of a rather thick quartz fibre 

 rendered conducting Ijy being dipped in calcium chloride solution, and 

 suspended between plates charged to equal and opposite potentials. The 

 potentials of the plates could be readily altered and the sensitiveness of 

 the electroscope varied at will. 



Possible Sources of Error. 



The method of protecting the receiving vessel from the Earth's electro- 

 static field has already been described. The best proof that the protection 

 was efficient is that when the vessel was disconnected from Earth, and no 

 rail! was falling, the electrometer-needle suffered no appreciable detiectiou, 

 even after a considerable interval of time. 



The most likely source of error in all such experiments on rain is the 

 possibility of splashing of the rain, from surfaces exposed to the Earth's field, 

 into the receiving-vessel. These drops would carry away an induced charge 

 opposite in sign to that of the gradient, and possibly of very considerable 

 magnitude, and would consequently prove a very potent source of error. It is 

 considered that in the apparatus used, the protection against this source of 

 error has been efficient. Rain hitting the edge or inner sides of the zinc 

 cylinder is caught on the sloping zinc cover below and drains off'. The rim 

 which has been turned up around the edge of this cover tends to prevent 

 splashing into the vessel below, which, even if it happens, is harmless so far 

 as introducing a spurious charge is concerned, inasmuch as the field at the 

 point is nil, owing to the protection afforded by the zinc cylinder. At 

 most it means that a few uncharged drops get into the receiving-vessel. In 

 general the apparatus may be said to have been protected from this effect 

 owing to the proximity of the buildings around it, which, as has been already 

 noted, to a great extent screened off the Earth's field. These buildings were 

 not, however, so near that any possible splashing could take place from any of 

 them into the receiver. 



Another possible source of error has to be considered, namely, the Lenard 

 effect. Lenard' has shown that, when pure water splashes on a metal surface, 



1 Wieii. Aimal., vol. xlvi, 1892, p. 626. 



