January 2, 1896] 



NATURE 



207 



A TMOSPHERIC ELECTRICITY.^ 

 TT is hardly possible to imagine that the first experimenter 

 -*■ who obtained an electric spark sufficiently strong to produce a 

 sensible sound should not at once have been struck by the fact 

 that he was in the presence of thunder and lightning on a small 

 scale. We find, indeed, in various writings from the early days 

 of electrical machines a number of suggestions that the thunder- 

 storm is an electrical phenomenon ; but to Benjamin Franklin 

 l)elongs the merit of having perceived that a direct experiment 

 was needed to prove what so far was only a guess. In an article 

 entitled "Opinions and Conjectures concerning the Properties 

 and Effects of the Electrical Matter arising from Experiments 

 and Observations made at Philadelphia, 1749," the following 

 passage occurs : — 



" To determine the question whether the clouds that contain 

 lightning are electrified or not, I would propose an experiment 

 to be tried where it can be done conveniently. On the top of 

 some high tower or steeple place a kind of sentry-box, big 

 enough to contain a man and an electrical stand. From the 

 middle of the stand let an iron rod rise and pass, bending out of 

 the door, and then upright 20 feet or 30 feet, pointed very sharp 

 at the end. If the electrical stand be kept clean and dry, a man 

 standing on it, when such clouds are passing low, might be 

 electrified and afford sparks, the rod drawing fire to him from a 

 cloud. 



" If any danger to the man should be apprehended (though I 

 think there would be none), let him stand on the floor of his 

 box, and now and then bring near to the rod the loop of a wire 

 that has one end fastened to the leads, he holding it by a wax 

 handle ; so the sparks, if the rod is electrified, will strike from 

 the rod to the wire, and not affect him." ^ 



The experiment suggested by Franklin was successfully per- 

 formed in Marly (France), by D'Alibard, on May 10, 1752,^ in 

 London by Canton, in Spital-square, on July 20, 1752, and by 

 Wilson, in Chelmsford, Essex, on August 12 of the same year. 

 Franklin himself describes having used a kite in Philadelphia in 

 a letter dated October 19, without giving the date of his observa- 

 tions. But this must be supplied in some passage which I have 

 not been able to find, for Rosenberger (" Geschichte der Physik," 

 vol. ii. p. 316) mentions that it was done in June. 



Franklin's disbelief in the dangerous character of the experi- 

 ment must have received a severe shock when he heard of 

 the death of G. W. Richmann, who, in the year 1753, was 

 killed by an electric discharge drawn from the clouds by means 

 •of a kite. 



The thunderstorm is the most impressive effect of atmospheric 

 electricity, though it is rivalled in beauty by the aurora, and in 

 interest by the many phenomena of daily occurrence, which are 

 only made perceptible to us by proper instruments. In a lecture 

 delivered before this Institution on May 18, i860. Lord Kelvin 

 •described the delicate electrical appliances constructed by him 

 for the more accurate observation of atmospheric electricity. 

 The problems then for the first time clearly stated, gave a 

 powerful and still lasting impulse to the investigation of atmo- 

 spheric electricity, and though no decisive answer can be given 

 to all the questions raised in that lecture, recent researches have 

 brought us somewhat nearer to their solution. 



Observations which may be made every day and at every 

 place have shown that the earth is electrified, whatever the 

 weather may be. In the language of the older theories, which 

 we cannot as yet altogether abandon, we say that the earth is 

 covered with negative electricity, or, in modern phraseology, we 

 express the same idea by the statement that we move about in 

 an electrified field, that electric lines of force stretch through the 

 air from the ground, from our bodies, and from everything which 

 is exposed to the sky overhead. The strength of this electric 

 field is not at all insignificant. If we wisli to produce it 

 artificially between two parallel plates kept at a distance of one 

 foot, we should have to apply an electromotive-force sufficient — 

 and sometimes more than sufficient — to light up the incandescent 

 lamps which illuminate our dwellings. The electric force is 

 comparatively weak in our country, but 50 volts per foot are 

 constantly observed, and icx) volts are not uncommon ; but in 

 dryer climates the amount of the force may be considerably in 

 excess of these figures. 



1 Discourse delivered before the Royal Institution of Great Britain, by 

 Prof. Arthur Schuster, F.R..S 



'^ "Experiments and Observations on Electricity made at Philadelphia, in 

 America," by Benjamin Franklin, LL.D. and F.R.S. (London, printed for 

 David and Henry, and sold by Francis Newbery, 1769, p. 66.) 



3 Ibid. , p. 107. 



If we fix our minds on the lines of force starting from the 

 surface of the earth, we are at once led to ask. Where is their 

 other end ? Do they curve round and back again to earth ? Do 

 they end in the dust which everywhere surrounds us, or do they 

 reach up to the clouds ? Do they pass through the clouds and 

 end where invisible particles separate the sunset red from the 

 midday blue ? Or, finally, do they leave the earth altogether, 

 and form intangible bonds between us and the sun, the stars, the 

 mfinity of space ? These are not idle questions, and we cannot 

 be said to have solved our problem unle.ss some definite answer 

 is given to them. The last-mentioned view, propounded origin- 

 ally by Peltier, and latterly supported by Exner, is the simplest. 

 If we could allow that the earth, once electrified negatively, 

 could remain electrified for ever, the corresponding positive 

 electrification being outside our atmosphere altogether, the chief 

 difficulty of atmospheric electricity would be removed, and the 

 normal fall of potential at the surface would be explained by the 

 permanent negative electrification of the surface. 



Unfortunately this view, to be tenable, has to assume that 

 the atmosphere is a complete non-conductor to the normal 

 electric stress, and this is known not to be the case. We know 

 of several causes which break down the insulating properties 

 of air. If two pith balls are electrified and repel each other, 

 and a match be lit in their neighbourhood, the pith balls come 

 together, showing that they have lost their charge, and conse- 



I! 



c c* 



quently that the flame of the match has destroyed the insulating 

 power of air. It is not only the flame itself which conducts, 

 but also the gases rising from the flame. ^ The following experi- 

 ment will prove this. In Fig. i, A represents a metallic tube 

 bent round at the upper end, and containing at its lower end a 

 Bunsen burner in metallic contact with the tube, which is also 

 connected to an electroscope. The tripod, T, which supports the 

 tube, is insulated by blocks of paraffin. A Leyden jar, L, on a 

 .separate support, is placed so that the knob stands at about the 

 level of the upper part of the tube, which acts as chimney to 

 the flame. The knob of the jar may be a few inches away from 

 the opening of the chimney, and not necessarily in a line with 

 it. The experiment succeeds, although the gases rising from 

 the burner may not come into contact with any part of the jar. 

 The jar is charged, and care must be taken that no fibres of 

 dust attach themselves either to the jar or chimney. I have 

 found it convenient to join a piece of amalgamated zinc to the 

 end of the chimney. Under these circumstances the charge of 

 the jar will be found to leak across to the tube, and the leaves of 

 it will diverge. If, as in Exner's form of electroscope, the 

 leaves, on reaching a certain divergence, discharge by forming a 



1 -The most complete investigation of the conduction of gases rising from 

 flames is contained in a series of papers by Giese {Wiedtmann's AnnaltH 

 vol. xvii.). 



NO. 1366. VOL. 53I 



