THE

LONDON, EDINBURGH, AND Dublin

PHILOSOPHICAL MAGAZINE

AND

JOURNAL OF SCIENCE.

[FIFTH SERIES.]

APRIL 1894.

XXXI. On the Electricity of Drops. By J. J. THOMSON, M.A., D.Sc., F.R.S., Cavendish Professor of Experimental Physics, Cambridge*.

DROPS

ROPS offer many advantages for the investigation of electrical effects, especially of those which involve the contact of dissimilar substances. Perhaps the greatest of these advantages is that when the drops are formed they present a perfectly clean surface to the gas by which they are surrounded; so that the conditions are much more definite than they can be where the surfaces have been long exposed, and have had time to get contaminated by dirt or coated with films of gas of unknown composition.

The experiments described in this paper relate to the electrical effects which occur when a drop of liquid falls on to a plate already coated with a film of the same liquid. It has been known for a long time that peculiar electric effects occur at the feet of waterfalls: at such places the normal distribution of the atmospheric electric potential is disturbed in such a way as to indicate a distribution of negative electricity in the region at the foot of the fall. These effects cannot be due to the waterfall acting like the falling drops in Lord Kelvin's water-dropping electrophorus; for they are found to occur when the waterfalls are inside caves whose sides are

*Communicated by the Author.

Phil. May. S. 5. Vol. 37. No. 227. April 1894. 2 A

dripping with moisture, in which cases the water would be falling inside an equipotential surface.

The subject of the electrification produced by falling drops has been investigated by Lenard, who gives the results of his researches in a most interesting paper, entitled "Ueber die Electricität der Wasserfälle" (Wiedemann's Annalen, xlvi. p. 584). In this paper Lenard shows that when drops of distilled water fall on to a plate wetted by the water, the drops after striking the plate are positively electrified, while in the air around the place where the drops fall there is a distribution of negative electricity: that no electrical separation takes place until the drops strike against the plate: and that the electrical effect with drops of tap-water is very much less than with drops of distilled water.

Lenard attributes these effects to the formation of a double coating of electricity over the surface of the drop, the drop itself being coated with positive electricity, while in the air close to the surface of the drop there is a coating of negative electricity. When the drop strikes against the plate some of this external coating gets knocked off, leaving the drop with a preponderating positive charge, the corresponding negative charge going off into the air near to where the drop strikes against the plate.

Some phenomena I observed when an electric discharge was passing through an exhausted tube led me to the conclusion that bodies were covered with this double coating of electricity, but that its moment and even its sign depended very much on the nature of the gas surrounding the body on which the coating was situated. I was therefore led to make a series of experiments on the phenomena which accompany the splashing of drops, especially on the effect produced by altering the gas through which the drops fall and the liquid of which the drops are made.

The methods I used are very similar to those employed by Lenard; they are represented in figs. 1 and 2. The arrangement shown in fig. I was used to test the effect when different liquids fell through air; that in fig. 2 when the drops fell through gases other than air. A is an insulated funnel from which the drops fall: the substance of which this funnel was made was not found to produce any effect upon the results. I tried funnels of glass, lead, brass, iron, and silver: the mouth of this funnel dips below the top of a metal tube connected to earth. In fig. 2 the top of this tube is closed by a paraffin stopper through which the funnel passes. The drops fall on the plate B, and after striking against it fall into the vessel C, which is insulated and connected to the liquid in the funnel and to one pair of quadrants of the

electrometer. The object of connecting the funnel to the vessel in which the drops are received is to eliminate the effect of any electrification that may be produced by the flow of the liquid through the funnel, or any electrical separation taking place when the drops fall from the spout of the funnel. It was found, however, by connecting the funnel by itself to the electrometer, that any effect of this kind was extremely small compared with those observed when both the funnel and receiver were connected with the electrometer. To prevent the electricity which goes into the air from charging the receiver, and thus neutralizing the opposite electrification given to the receiver from the drops, different methods were employed according as the drops fell through air, when the experiments could be made in the open, or through other gases, when the apparatus had to be enclosed in an air-tight vessel. In the first case the separation of the two electricities was effected by blowing a current of air across the plate on which the drops fell; the electrification in the air was by this means blown against a metal plate connected with the earth, and so discharged. The current of air was produced by a fan worked by a water-motor. This method could not be used when the apparatus had to be closed up so as to allow of gases other than air being used. In this case the electrification in the air was discharged by placing over the plate a piece of wire gauze connected with the earth: a hole was cut through the gauze to allow of the drops falling through without touching the gauze; the gauze was separated from the plate by a distance only just great enough to allow the drops to fall into the receiver after rebounding from the plate without striking against the gauze. Plates of various kinds were tried, but I found, as Lenard observes, that the material of which the plate is made exerts no influence provided it is thoroughly wetted by the liquid which falls upon it. Irregularities occur if the plate is greasy, when the drops, instead of spreading over the plate, roll off it, still retaining their globular form.

With either of these pieces of apparatus the effect discovered by Lenard, that after striking against the plate drops of distilled water had a positive charge, could easily be observed.

The first variation of the experiment which I tried was to make the drops fall through water-vapour instead of air. For this experiment water was boiled for several hours in a vessel like that represented in fig. 2 until the steam had completely expelled the air: the water used for the drops had been well boiled so as to thoroughly expel the air from it. In this case no electrical separation whatever was observed, though when air was blown into the vessel the normal effect at once reappeared.

funnel to the vessel inate the effect of by the flow of the separation taking of the funnel. It nel by itself to the is extremely small h the funnel and eter. To prevent om charging the site electrification ent methods were ugh air, when the or through other ed in an air-tight e two electricities

ross the plate on he air was by this ed with the earth, produced by a fan ould not be used s to allow of gases electrification in e plate a piece of was cut through h without touchm the plate by a the drops to fall he plate without ious kinds were the material of e provided it is pon it. Irregu drops, instead retaining their

When the vessel was filled with hydrogen, the electrification of the drops was reversed: the drops, after striking against the plate, being negatively electrified, while there was a positive charge distributed through the hydrogen. Lenard found that the effect was considerably less in hydrogen than in air. If care is taken to get rid of all traces of air, the effect is not merely diminished but reversed.

This and the experiment previously described seem to indicate that the effect is due to chemical action. This view is confirmed by the very large effects produced by the addition of small quantities of various foreign substances to the water forming the drops.

The results of experiments made upon a considerable variety of substances are given below. In each case the deflexion given is that produced when 100 cubic centim. of the substance fall through the funnel. The deflexion is marked + when it is in the same direction as that due to pure water falling through air.

In this case any addition of the substance diminishes the electrical effect, and only requires a small quantity of HCl to make the solution neutral. When the solution has become neutral it remains so in spite of the addition of more acid. We shall see that other substances, such as zinc chloride, oxalic acid, &c., show the same peculiarity.