208 



NATURE 



[January 2, 1896 



contact with earth-connected plates, c c', the charging and dis- 

 charging can be watched for a long time. It will be noticed 

 that the flame, being altogether surrounded by a tube of the 

 same potential, cannot be active in this case, but the conduc- 

 tivity must be due to the gas as it escapes from the chimney. 



It follows from these experiments that every fire burnt on the 

 surface of the earth, and every chimney through which products 

 of combustion pass, act like very effective lightning conductors, 

 and would consequently discharge, slowly but surely, any electri- 

 fication of the surface of the earth. The peculiar immunity of 

 factory chimneys against damage by lightning appears from 

 statistics collected by Hellmann in Schleswig-Holstein,^ for 

 while 6'3 churches per thousand were struck, and 8'5 wind- 

 mills, the number per thousand of factory chimneys was only o"3. 



Franklin was acquainted with the action of flames ; he also 

 discovered that no charge can be given to a red-hot iron ball, 

 a fact which seems to have been forgotten until re-discovered in 

 our own times by Guthrie. Franklin also tried the action of 

 sunlight^ but obtained no result. Had he performed the experi- 

 ment with carefully-cleaned zinc, he would have anticipated one 

 of the most striking of Hertz's discoveries. We now know that a 

 negatively-charged surface will discharge into air when illumin- 

 ated by strong violet light, and sunlight will be sufficient with 

 specially sensitive materials. This action has been investigated 

 in detail by Elster and Geitel, who have not, however, succeeded 



LJ-1 



Earth. 



Fig. 



in obtaining results with sunlight acting on such bodies as we 

 know the earth's crust to be made of So far, then, we have no 

 experimental evidence to include light as an active agent in the 

 phenomenon of atmospheric electricity. 



We possess in the electric discharge itself a very powerful, 

 and probably very generally active means of breaking down the 

 insulating power of air. Some of the experiments [Proc. Roy. 

 Soc. , vol. xlii. ) which I described some years ago to prove this, 

 were objected to on the ground that it might not be the discharge 

 itself, but the ultra-violet light sent out by the luminosity of the 

 discharge, which was active. The following form of the experi- 

 ment conclusively shows that the discharge acts independently 

 of light. 



In Fig. 2, R represents a Rhumkorff coil entirely surrounded 

 by a metallic box, B, which is connected to earth. The terminals 

 of the coil lead to two electrodes inside a metallic tube, T, which 

 is also kept at zero potential. This tube is arranged so that a 

 current of air can be blown through it. The air, on escaping 

 through the tube, is made either to impinge on or to pass near 

 a metallic plate connected to a charged electroscope. Under 

 these circumstances, the electroscope is not discharged 

 either by a current of air alone, or by the coil alone. But 

 as soon as the air is blown through the apparatus while 

 the sparks are passing and then made to impinge on the 

 plate c, the electroscope is instantaneously discharged. The 



1 " Veroffentl. des Kgl. Preuss, Stat. Bureaus," 1886, p. 177, quoted by 

 Bebber, " Meteorologie," p. 245. 



experiment succeeds when a plug of cotton-wool is inserted at w^ 

 to stop the action of the dust ; but a plug of cotton-wool at the 

 other end diminishes the action so much, that I am doubtful 

 whether the effect then really exists there. I am, so far, not 

 inclined to believe that the action is due to dust, but rather that 

 the cotton-wool acts in increasing very considerably the interval 

 which elapses between the time at which the spark acts and the 

 time at which the Sparked air passes out of the tube. The effect 

 may be observed even though the tube L is lengthened by an 

 addition of another piece 3 feel or 4 feet long. 



Several phenomena, one of which had been known for a long 

 time, can be explained by the fact that the electric discharge 

 changes the condition of the gas into a state similar to that of 

 gases rising from flames. It is mentioned, for instance, by 

 Faraday that electric sparks are liable to succeed each other along 

 the same path, and it is known that the same holds for lightning 

 flashes, facts which themselves point to a higher conductivity of 

 air along the path of the previous discharge. A curious 

 instance of a similar effect is afforded by lightning conductors, 

 which are sometimes put up to protect overhead leads used for 

 conveying a high tension current. Owing to the obvious 

 impossibility of connecting the leads directly to earth, a small 

 air gap is interposed, the idea being that the air gap will act as 

 an insulator for the current the leads are intended to carry, but 

 that if during a thunderstorm the potential rises sufficiently high 

 to be dangerous, equalisation may take place through the air 

 gap to earth by means of a small spark. So far, the air gap 

 answers its purpose, but as soon as a spark passes through 

 the gap, it destroys the insulating power of the air, and the 

 main current consequently takes a short-cut through the gap. 

 At Pontresina, in the Engadine, lightning conductors put up ini 

 this way are so sensitive that a flash of lightning several miles 

 away causes a small spark by induction, and instantaneously 

 puts out every electric lamp in the town. 



If we accept the view that an electric discharge destroys the 

 insulating power of the gas, it follows that the outer regions of 

 the atmosphere must conduct, for we have ample reason to 

 suppose that electric currents are passing continuously through, 

 those regions. The aurora borealis in the arctic regions is,, 

 according to Nordenskiold's observations, a permanent phen- 

 omenon, and the diurnal changes of terrestrial magnetism show 

 that in our latitudes electric currents traverse the air above us» 

 However small a conductivity we may assign to the atmosphere, 

 the earth could not remain electrified" inside such a shell of 

 partially conducting gases. Lord Kelvin drew the same con- 

 clusion in the Royal Institution lecture, on the assumption that 

 gases at much reduced pressures cease to insulate. We may 

 leave it an open question whether the normal electric stress 

 could in itself cause a discharge in the outer regions ; but we 

 cannot deny that under existing conditions these regions do not 

 insulate, and Lord Kelvin's argument still holds good. 



But the question of the ending of the lines of force — in other 

 words, the location of the positive charge corresponding to the 

 negative electrification of the surface of the earth — can only be 

 solved by balloon or kite experiment, and we may briefly 

 mention the more important results which have so far been 

 obtained. 



Observations made up to heights of about 1000 feet seem to- 

 indicate a strengthening of the electric field — i.e. the fall of 

 potential per metre is greater at a height of, say, 200 metres 

 than on the surface of the earth. The observations of Dr. 

 Leonhard Weber {ElektrotecJmische Zeiischriff, April 1888) 

 bring out this point clearly. In one case the fall of 

 potential at a height of 350 metres was found to be six times 

 that at the earth's level. This increase is in itself not 

 surprising, if we remember that every particle of dust 

 raised from the ground must itself be negatively electrified, and; 

 probably the observed increase in the electric force is sufficiently 

 accounted for by the presence of electrified dust. 



Observations made at greater heights in balloons, on the other 

 hand, seem clearly to indicate that this increase soon ceases^ 

 and that a diminution already takes place at moderate heights. 

 Thus the observations of Dr. O. Baschin {Meteorologische 

 Zeitschrift, September 1894) gave for the fall of potential in 

 volts per metre the numbers 49, 28, 13 at heights of 760^ 

 2400, 2800 metres respectively, and at a height of 3000 metres 

 no measurable fall at all could be obtained. These observa- 

 tions were made in clear weather. The balloon afterwards 

 passed over a layer of clouds, and strong electric effects were- 

 noticed. Similar observations had been previously made by 



NO. 1366, VOL. 53] 



