January 2, 1896] 



NATURE 



211 



the various bodies present in the atmosphere/ The possibility 

 of contact electricity between a solid or liquid and a gas, is 

 not quite easy to submit to the test of experiment. If we rub 

 two solid bodies together, %e may, by separating them, investi- 

 gate the electric field produced ; but, supposing we have a drop 

 of water surrounded on all sides by air, the water may be 

 covered with an electric layer of, say, positive electricity, the air 

 in contact with the water with the opposite kind, and it is not 

 at all clear how we could experimentally demonstrate the differ- 

 ence of potential between the air and the drop which is thus 

 produced. A current of air flowing past the drop might carry 

 away some of the negative layer, and in this way an electric 

 field may be established while clouds are forming, but the con- 

 ditions necessary for an experimental demonstration would be 

 very difficulty to realise. Two methods have been devised which 

 practically demonstrate some form of contact electricity between 

 gases and water. 



Lenard, wishing to imitate the electric field observed in the 

 neighbourhood of waterfalls, has established by careful experi- 

 ment a number of important facts, which are all consistent with 

 the following explanation. If we imagine two oppositely elec- 

 trified layers at the surface of a drop of water such as has been 

 referred to, and if the drop falls on to a layer of the same liquid, 

 or if similar drops impinge on each other, the difference of 

 potential produced by the fusion of the surface layers becomes 

 greater than is consistent with equilibrium. For, taking the 

 case of drops falling into a mass of water contained in a 

 cylindrical vessel, the extent of surface between air and water is 

 not increased by the falling drops, and we must imagine that 

 surface to be already covered with a sufficient electrical sheet to 

 establish the required difference of potential, The electrification 

 of the drops is, therefore, not wanted, and a change in the dis- 

 tribution takes place. The natural supposition would be, that 

 this equilibrium would be restored very quickly through the 

 surface of the water, but a certain time seems to be required for 

 this. Meanwhile, the strong current of air which in Lenard's 

 experiments is brought down with the water drops carry some 

 of the electricity away, the water remaining positive. More 

 recent experiments of Lord Kelvin's, with air bubbling through 

 water, point similarly to contact forces between gases and 

 liquids, and in these experiments also it appears that a consider- 

 able time is required to establish electric equilibrium between a 

 gas and a solid. Lenard finds very important differences caused 

 by small impurities in the water, the water acting much more 

 strongly when it is pure. If it contains as much salt as is con- 

 tained in the sea, the effect is reversed, and the air becomes 

 positively electrified. The explanation which is given above is 

 practically that of Lenard, whose observations have been con- 

 firmed and further extended by Prof. J. J- Thomson. These 

 experiments, no doubt, account for the behaviour of air in the 

 neighbourhood of waterfalls, and they probably also explain 

 the negative electrification of air in the neighbourhood of dis- 

 tricts in which rain is falling. The strong positive electrification 

 of mist may also be due to the same cause. 



There seems to be no doubt that the formation of a cloud is 

 often accompanied by electrical effects. A few years ago, 

 descending from the Dent Blanche, I found myself, after sunset, 

 at a height of about 12,000 feet. A current of air was apparently 

 blowing up the valley which stretches from Evolena towards 

 Ferpecle, and I could observe a cloud condensing below me at 

 a height a little below the snow-line. As night came on and 

 we continued our descent over the glacier and down the valley, 

 a series of electric discharges were noticed between the cloud, 

 which was lying in a deep-cut valley, the sides of the mountain, 

 and the blue sky overhead. Here the moist air was evidently 

 streaming through the cloud, depositing its moisture in the form 

 of drops, and it seemed the most natural explanation at the 

 time that the air left the cloud in an electrified state. 



But while by means of experiments we have been able to 

 produce some of the phenomena of atmospheric electricity, we 

 have other important effects which cannot be accounted for in so 

 simple a way. The electric discharges during a thunderstorm 

 give evidence of electric fields, which could hardly be explained 

 by contact electricity between drops of water and air alone. 

 The fact that thunderstorms are nearly always connected with 

 the formation of hail, and Faraday's experiments showing that 

 water rubbing against ice becomes negatively electrified, is made 

 use of in the theories of Sohnke and Luvini. It is quite likely 

 that there is some truth in these theories. Their weak point 

 lies in the difficulty of seeing how particles of ice and water 



NO. 1366, VOL. 53] 



can be first sufficiently mixed to allow of friction, and then 

 become sufficiently separated to produce an electric field of 

 such magnitude as we know must exist in a thunder cloud. 



It is to be remarked, however, that the laws of contact 

 electricity must be applicable to gases as well as to solids, and 

 that if water becomes positive when rubbing against air, and 

 negative when rubbing against ice, there must be a strong 

 contact difference between ice and air. In other words, it does 

 not matter whether there is direct friction between ice and water, 

 or whether the air forms an intermediate body. We may 

 imagine air rising through a cloud containing drops of water 

 negatively electrified, and then passing through an ice cloud 

 having its negative electricity increased, thus leaving the ice and 

 water particles at a difference of potential which may, by a 

 fusion of the drops, increase sufficiently to produce a lightning 

 discharge. This seems to me the most plausible theory which, 

 in the present state of our knowledge, can be formed. As 

 regards the permanent negative charge of the earth's surface, 

 the time has not yet arrived for forming a definite opinion. 

 Although we know that the earth, once electrified, would 

 gradually lose its charge into the atmosphere, yet we can express 

 no opinion as to the rate at which the loss is going on. That 

 loss may be exceedingly slow, and consequently equilibrium 

 might be attained by a very small preponderance of negative 

 electricity brought back to its surface through some cause or 

 other. Rain, as has already been mentioned, is more frequently 

 electrified negatively than positively in our own climate, and 

 though we do not know how far this holds in the tropical belt, 

 it is at any rate possible that the surface of the earth may in this 

 way alone make up for the loss. We may also reasonably think 

 that Lenard's observation on salt water may account for the 

 permanent charge. Every wave that breaks into spray under the 

 action of a strong wind would leave the water negatively electri- 

 fied, the air carrying away the positive charge. It would be of 

 great interest to possess observations on atmospheric electricity 

 on board ship while waves are breaking in the neighbourhood. 

 So far we have only Exner's observations to guide us, who 

 found, while observing at Lavinia, in Ceylon, that the spray 

 from breaking waves affected the indications of the electrometer, 

 proving its positive electrification ( IVienerAkad. Sitztingsberichte, 

 vol. xcviii.). 



But although the loss of electricity from the earth's surface 

 may be very slow, it is equally possible that it is considerable. 

 We shall not be able to treat this question satisfactorily until we 

 have some clearer notion of the causes of the aurora. We know 

 that the aurora implies electric currents, and the circuit of these 

 currents may lie completely within the earth's atmosphere, and 

 haye nothing to do with the observed fall of potential near the 

 ground. It is also possible that the body of the earth forms part 

 of the electric circuit, and if that is the case, there must be 

 across different parts of the surface an outward and inward flow 

 of positive electricity. Such a discharge could not fail to 

 influence the phenomena we have discussed, and it seems probable 

 that we should have some evidence derived from observ-ation if 

 the aurora was always accompanied by discharges through the 

 earth's surface. Except in the polar regions, these aurorre do 

 not seem to affect the normal fall of potential. There is a third 

 view we may take as to the circulation of electric currents 

 indicated by the aurora : the return current may take place in 

 space outside the earth's atmosphere. A good deal might be 

 said in favour of this view, and the rotation of the earth's 

 magnetic field in space might be a sufficient cause for the 

 production of these currents ; but this is not the place to enter 

 further into this question. 



Calculations made from observation on the height of the 

 aurora have generally resulted in an altitude of from lOO to 200 

 miles, except in the polar regions, where the aurora seems 

 occasionally to descend to a much lower level. It has also been 

 noticed that aurora; are associated with certain bands of cirrus 

 clouds, and this seems to indicate that although the luminous 

 phenomenon is sufficiently intense to be observed at only great 

 heights, yet the electric phenomena may descend to the level of 

 the cirrus. 



As regards the connection between the aurora and the sun-spot 

 period, further observations in the polar regions are needed. On 

 the one hand, we have Paulsen's^ statement, derived from 

 observations in Greenland, to the effect that the greatest number 



1 Paulsen, "Danske Videnskab. .Selskabs Forhand.," 1880. (I have not 

 seen the original memoir, but only an abstract in the Jahrbuch der AstrO' 

 nomie und Gecphysik, 1890.) 



