108 Dr. C. Chree. Observations on Atmospheric 



Defects in Water-dropper and Portable Electrometer. 



11. Both instruments aim at communicating the potential at a 

 fixed point in the air to an insulated conductor by detaching from a 

 mass in electrical connexion with the conductor a continuous succes- 

 sion of small elements. It is at least doubtful whether either instru- 

 ment can ever fully accomplish its object. If the object were so far 

 accomplished that a constant fraction of the true potential were 

 recorded, the deficiency of this fraction from unity would hardly be 

 of primary importance in dealing with diurnal or annual variation ; 

 but if the fraction has itself a diurnal or secular variation it is a very 

 different matter. 



In the water-dropper a uniform state of insulation of the water- 

 can, electrometer needle, and connecting wire is not easy. Absolute 

 insulation, when a voltage runs up to hundreds, is a somewhat ideal 

 state of perfection. When the insulation is indifferent, the recorded 

 may fall far below the true potential. The water jet, so to speak, is 

 running up the potential, leakage from the can, wire, &c., running it 

 down. The resultanfc effect depends on a variety of things, e.g., the 

 rate at which the air potential is changing and the supply of water 

 particles. Unless the potential is unusually steady, and the insula- 

 tion exceptionally good, one may expect higher potential records with 

 a copious jet than with a restricted one. 



In the portable electrometer there' is similarly some ground for 

 expecting the potential recorded to be influenced by the rate of com- 

 bustion of the fuse. 



The uniformity of the disintegrating mass may also be of import- 

 ance. With a water-dropper there ought not to be much uncertainty 

 on this ground, but as electrometer fuses are articles of commerce 

 uniformity in their material and condition is less easily ensured. 



There is a final source of uncertainty common to the two instru- 

 ments as commonly used. With the water-dropper the spot where 

 the jet breaks up is apt to be slightly influenced by variations in the 

 water pressure. When the issuing jet makes as usual an angle with 

 the wall of a building, the consequences, as appears from Table II, are 

 likely to be appreciable. It was a recognition of this fact that led to 

 the taking of the observations with the portable electrometer at two 

 nearly fixed hours, the afternoon one when the can was nearly full, 

 the forenoon one when it was about half empty. 



The corresponding defect with the portable electrometer is the 

 burning down of the fuse. When the fuse is used in a vertical 

 position, the height of the spot whose potential is being measured 

 diminishes as the fuse burns, and with the height the potential falls 

 off. 



No direct comparison of the readings of the two instruments at one 



