Page:Proceedings of the Royal Society of London Vol 60.djvu/123

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§11. Both instraments 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 succession of small elements. It is at least doubtful whether either instrument 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 watercan, 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 resultant 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 insulation 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 combustion of the fuse.

The uniformity of the disintegrating mass may also be of importance. 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 instruments 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