Aug. 15, 1884.] 



- KNOAVLEDGE • 



131 



LIGHTNING 



THE HISTORY OF A 

 FLASH. 



By W. Slisgo. 



LATELY -we have all felt, I doubt not, a considerable 

 amount of interest in the various phenomena attend- 

 ing this summer's unusually heavy thunderstorms, accom- 

 panied, as thej have been, by vivid lightning discharges of 

 a more or less hurtful nature. The list of disasters pub- 

 lished in Knowledge, No. 14.3, might be very materially 

 augmented were we to record such damage as has been 

 wrought since that list was compiled. 



There is not, I suppose, in the mind of any intelligent 

 man at the preseut day a doubt as to the electrical origin 

 of a lightning flash. The questious to be considered are 

 rather whence comes the electricity ? and in what way is 

 the thunderstorm brought about ] In attempting to 

 answer these questions, sight must not be lost of the fact 

 that the very nature of electricity is in itself almost suffi- 

 cient to baffle any effort put forth to ascertain from light- 

 ning, as such, its whence and its whither. 



It is possible, however, with the aid of our knowledge of 

 static electricity, to arrive at hypotheses of a more than 

 chimerical nature. In the first place, that our sphere is a 

 more or less electrified body is generally admitted. Jlore 

 than this, it is demonstrated that the different parts of the 

 earth's surface and its enveloping atmosphere are variously 

 charged. As a consequence of these varying charges, there 

 is a constant series of currents flowing through the various 

 parts of the earth, which show themselves in such telegiaph- 

 wires as may lie in the direction followed by the currents. 

 Such currents are known as earth-currents, and present 

 phenomena of a highly interesting nature. But, apart 

 from these electrical manifestations, there is generally a 

 difference of electrical condition between the various parts 

 of the earth's surface and those portions of the atmosphere 

 adjacent to or above them. Inasmuch as air is one of the 

 very best insulators, this difference of condition (or po- 

 tential) in any particular region is in most cases incapable 

 of being neutralised or equilibriated by an electric flow. 

 Consequently the air remains more or less continually 

 charged. With these points admitted as facts, the question 

 arises. Whence this electricity 1 There have been very many 

 and various opinions expressed as to the cause of terrestrial 

 electricity, but far the greater portions of such theories lack 

 fundamental probability, and indicate causes which cannot 

 be regarded as sufficiently extensive or operative to produce 

 such tremendous effects as are occasionally witnessed. I 

 take it that we may safely regard the evolution of elec- 

 tricity as one of the ways in which force exhibits itself, 

 that, in other words, when work is performed electricity 

 may result. When two bodies are rubbed together, elec- 

 tricity is produced, so also is it when two connected metals 

 are immersed iu water and one of them is dissolved, or when 

 one of the junctions of two metals is raised to a higher 

 temperature than the other junction. I will go further 

 than this, so far, in fact, as to maintain that there is 

 reasonable ground for supposing that every movement, 

 whether it be of the mass or amongst the constituent par- 

 ticles, is attended by a change of electrical distribution, and 

 if this is true it may easily be conceived that inasmuch as 

 motion is the rule of the universe, there must be a constant 

 series of electrical changes. Xow, these changes do not all 

 operate in one diiection. nor are they all of similar character, 

 whence it is that not only are there earth currents of 

 feeble electro-motive force, but that this E M F is con- 

 stantly varying, and that, furthermore, electricity of high 



E JI F is to be met with in various parts of the atmo- 

 sphere. 



With earth currents we have here very little to do. The 

 rotation of the earth is in itself sufficient to generate small 

 currents, and the fact that they vary in strength at regular 

 periods of the day and of the year enforces the suggestion 

 that the sun exerts considerable electrical influence ou the 

 earth. Letting it be granted, however, that the earth is 

 variously charged, how comes it that the air is also charged, 

 and with electricity of greater tension than that of the earth 

 itself 1 It was pointed out by Sir W. Grove that if the 

 extremities of a piece of platinum wire be placed in a 

 candle-flame, one at the bottom and the other near the top, 

 an electric current will flow through the wire, indicating 

 the presence of electricity. If an electrified body be heated, 

 the electricity escapes more rapidly as the temperature 

 rises. If a vessel of water be electrified, and the water 

 tlien converted into steam, the electric charge will be rapidly 

 dissipated. If a vessel containing water be electrified, and 

 the water allowed to escape drop by drop, electricity wpl 

 escape with each drop, and the vessel will soon be dis- 

 charged. We regard it as an established fact that the 

 earth has always a greater or less charge ; whence it is safe 

 to assume that in the process of evaporation wliich is going 

 on all over the surface of the globe, more particularly in 

 equatorial regions, every particle of water, as it rises into 

 the air, carries with it its portion, however minute 

 that portion may be, of the earth's electric charge. 

 This small charge distributes itself over the surface of the 

 aqueous particle, and the vapour rises higher and higher 

 until it reaches that point above which the air is too rare 

 to support it. It then flows away laterally, and as it 

 approaches colder regions, gets denser, sinking lower and 

 nearer to the eartli's surface. The aqueous particles 

 becoming reduced in size, the extent of their surfaces is 

 proportionately reduced. It follows that as the particles 

 and their surfaces are reduced, the charge is confined to a 

 smaller surface, and attains, therefore, a greater " surface 

 density," or, in simpler language, a greater amount of elec- 

 tricity per unit of surface. Electricity, as above eet forth, 

 is in what is known as the "static" condition (to di>tinguish 

 it from electricity which is being transferred in the form of 

 a current), when it has the property of " repelling itself " 

 to the utmost limits of any conductor upon which it may 

 be confined This will account for the charge finding its 

 way to the surface of the water particles, and will further- 

 more account for the greater density of the charge as the 

 particle gets smaller and has the extent of its surface 

 rapidly diminished. It may be mentioned that the surface 

 of' a sphere varies as the cube of its radius. Returning 

 to the discussion of the state of affairs existing when the 

 particles have reached their highest position in the atmo- 

 sphere, we may imagine that they set themselves off on 

 journeys towards either the north or the south pole. As 

 they pass from the hotter to the colder regions, a number 

 of particles coalesce ; these again combine with others on 

 the road until the vapour becomes visible as cloud. The 

 iiicreased density implies increased weight, and the cloud 

 particles, as they sail pole-wards, descend towards the 

 surface of the earth. Assuming that a spherical form is 

 maintained throughout, the condensation of a number of 

 particles implies a considerable reduction of surface. Thus, 

 the contents of two spheres vary as the cubes of their 

 radii, or eight (the cube of 2) drops on combining will 

 form a drop twice the radius of one of the original drops. 

 We may safely conceive hundreds and thousands of such 

 combinations to take place until a cloud mass isformed, in 

 which the constituent parts are more or le^^s in contact, 

 and, therefore, behave electrically as a single conductor of 



