Page:Proceedings of the Royal Society of London Vol 60.djvu/113

98 § 2. The first question is : supposing the apparatus perfect, does the electrograph supply information as to the potential anywhere except at the spot where the stream of water breaks into drops ? To answer this question, one has to consider the influence of the environment, notably the proximity of a lofty building.

An investigation into this point was made ten years ago by Professor Exner, of Vienna, who found the equipotential surfaces near a building much deflected from horizontality. His results indicated apparently that for practical purposes the whole building might be regarded as possessing the earth’s potential. Whilst it was anticipated that Exner’s conclusions vrould hold good of Kew Observatory, it appeared prudent as a check to take observations with the portable electrometer, at a series of points in a vertical plane perpendicular to the west wall near the water-dropper. Observations were taken at heights of 3, 6, and 9 feet from the ground, which possesses, it may be explained, a slope away from the building. The base line, starting at the Observatory wall, terminated 57 feet away in a parallel wall 11 feet high, belonging to a much lower building. The observations were repeated on several days, but one example will suffice. The potential measurements are in volts, the distances from the Observatory wall in feet.

Mean Distance from wall............. 3 6 12 18 24 30 36 42 48 54 potential. Potential at height 3 fe e t.. 4 6 18 38 48 46 34 24 16 6 26 6 „ .. 8 18 40 58 88 84 76 68 52 22 56 ” 9 _ 28 44 76 102 120 120 108 68 36 78

In forming the means in the last column the results at 3 feet from the Observatory wall were omitted. The readings were uncorrected for variations of potential during the interval occupied by the observations.

So far as they go, the results are clearly confirmatory of Exner’s. They show that the influence of a tall building in pulling down the potential extends to a considerable distance.

§ 3. The large dependence of the electrograph records on the immediate environment of the water jet complicates matters, but this need not prove a serious obstacle if the conditions allow us to regard the problem as one of statical electricity, in which influencing bodies are either stationary or at a distance. On this hypothesis, simultaneous potentials at any two neighbouring points would stand to one