♦ KNOWLEDGE ♦ 



[JcLT 24, 1885, 



discliarged, three sparks will be seen between these balls. 

 Looked at under ordinary conditions, the three sparks 

 will appear simultaneously. 



Opposite the balls, however, was placed a revolving 

 mirror, making 800 revolutions per second, and the 

 images of the sparks received by it were reflected on to a 

 screen. It was observed on discharging that the images 

 appeared in the manner shown in Via. 2, A and F being 

 simultaneous, while C was a liitir lati r. The retarda- 

 tion of the middle line or spark wiis i;i!culutud to amount 

 to one 1,152,000th of a second: tl.:>i i^to s;,v, this was the 

 time taken by the dischnr-.' tn tr.^v, rsr x]',- nuurtcr of a 

 mile of wire between 1! .m.I C , r K ai.l D. .-i-rt .-pond- 

 ing, in tJas ^>a, -tic Ilia r :■■■.■■. M ;. \rl^chv nf i>s.(iOO mik-s 

 per second. From tliv KiiLrths of the" aivs. \shirh were 

 in all three cases identical, it was calculated that the 

 duration of each spark was one 24,000th of a second. Had 

 the discharge travelled from the positive plate P to the 

 negative plate N, then a flash of lightning would have 

 been seen pictured on the screen between each pair of 

 balls, that between A and B occurring first, and the other 

 two following in rapid succession. Had the discharge 

 started at the negative plate the flashes would have 

 been seen in the reverse order. Had there been (on the 

 double-fluid hypothesis) a mutual exchange, Wheatstone 

 would have observed, not three, but at least five sparks 

 pictured, two occurring between A and B, two between 

 F and E, and one between C and D. 



The evident teaching of this experiment is that a 

 lightning discharge may start from earth and cloud 

 ■nmultaneoiisly, meeting at some central point. But ai-e 

 -we jastified in inferring that it always does so 1 I 

 think not. I rather imagine that in the majority of 

 cases the conditions are not altogether comparable with 

 those of the experiment. The whole matter turns upon 

 the question of electrical '• tension," a term which may 

 be defined as the tendency to produce an electrical dis- 

 charge. Where, over :i _ i , e : : rr i. there is the greater 

 ■concentration of tl. .■ , ' v, ill be, perforce, the 



greater tension. Tin ■ : < f electricity on a con- 



ductor is rarely umfdi ,1. \ji\iiil' \iry considerably with 

 the shape. "Wliere the greater pr.iminences occur, there 

 will be found the greater concentration of electricity, 

 whence it is said that electricity accumulates at points ; 

 and it is well known to the merest tyro in electrical 

 science that this concentration at points results in a pro- 

 portionallj- increased tendency to produce a discharge. 

 If, then, a certain quantity of electricity is con- 

 fined to a cloud, and that quantity, acting inductively 

 upon the subjacent earth,' produces an equal degree of 

 electrification (of the opposite kind), it follows that^ in the 

 majority of cases, the charge on the cloud will be at a 

 greater tension than that on the earth, for the simple 

 reason that, more often than not, the cloud is more pointed 

 or irregular in shape than the earth. As a matter of fact, 

 the terrestrial charge is more or less uniformly distri- 

 buted throughout the neighbourhood, while in the cloud 

 there is a considerable concentration at the edge or pointed 

 portion. Whence it ensues that, in the majority of cases, 

 there is great reason to suppose that the discharge strikes 

 downwards only. On the other hand, were the ter- 

 restrial surface provided with but a single point, such as 

 is furnished by the well-known lightning protector, the 

 discharge would only take place upwards. Moreover, 

 the discharge would be a gradual, or, within certain 

 limits, a continuous one, because the tension would grow- 

 so rapidly {as the cloud approached) that it would be 

 quite incapable of retaining the charge. Had Wheat- 

 stone been able so to modify his experiment as to place 



at A or F a huge sphere, or even a plate instead of a 

 small ball, and, converselj-, to substitute for F or A a still 

 smaller ball approaching even a point, there is to my 

 mind little doubt but that he would have seen a different 

 result. He would in all probability have obtained the 

 sparks in succession, that near the small ball fir.st, followed 

 by one in the centre, and another striking the large 

 surface. Now neither the single nor the double-fluid 

 theories can adequately account for this. 



We may, however, regard the charged cloud and the 

 subjacent electrified earth as the extremities of a long 

 elastic chain of electrically-polarised particles, each sub- 

 jected to a series of stresses increasing in strength until 

 it is compelled to yield. This chain, indeed, strives to 

 shorten itself, and it is this striving that we know as 

 electrical attraction. With the necessary facilities this 

 shortening takes place, chief amongst these facilities 

 being the physical elasticity or mobility of the intervening 

 substance (or di-electric) — in this case, air — and the low 

 electrical capacity of the charged bodies.* 



We may compare the polarised chain to a rod of 

 extended elastic held at the extremities until the exten- 

 sion is sufficiently great to overcome the liolding or 

 restraining power. If two oppositely-charged bodies 

 have equal capacity, we can conceive that, when the dis- 

 charge occurs, it will bear some resemblance to a piece 

 of ext. ii.L il eliisti- set free at both ends simultaneously. 

 Thi.-, i- . XHtly uliiit happened in Wheatstone's experi- 

 ment. A\"Iii r.-' ill. two bodies have different capacities, 

 we iiKiy c-siii.nn- the jiolarised chain to a piece of elastic 

 held niore firmly at one end than at the other, whence it 

 follows that the contraction will be one-ended, that 

 which is less securely held corresponding to a charged 

 body of small capacity, and which, therefore, speedily 

 attains to the tension necessary to produce discharge. 



In conclusion, then, the lightning discharge may take 

 place (i.) simultaneously from earth to cloud and cloud 

 to earth, (ii.) from earth to cloud, (iii.) from cloud to 

 earth. The first form happens when the configuration 

 of earth and cloud are (electrically) similar ; the second 

 when the earth is sharper than the cloud ; the third 

 when the cloud is more pointed than the earth, and it is 

 this third form which I think most frequently ensues. 



EA31BLES WITH A HA^IMER. 



Br W. Jerome Harbison, F.G.S. 

 FEOil NUNEATON TO TAMWOETH— THE WARWICK- 

 SHIRE COAL-FIELD. 

 {Continued from p. 21.) 



BUT we have not yet seen the finest exposure of the 

 Cambrian shales. If, on leaving Hartshill, we 

 direct our steps in a south-westerly direction to Stock- 

 ingford Station (2| miles), and then walk along the 

 Midland line to Nuneaton, the following facts may be 

 observed. Soon after leaving the station we note on the 

 left-hand side of the line a large brick-pit, showing coal- 

 measure shales and sandstones dipping at liii;-h angles to 

 the south-west, and much disturbed, indicative of a fault 

 (missed by the survej-ors), which runs between the coal- 

 and the Cambrian beds beneath. This line of 



* Capacity U measured by the quantity of electricity required 

 to raise the charge on a conductor to a given potential. A body 

 which has the highest capacity is that which combines minimum 

 surface with maximum content, viz., a sphere. In a body having, 

 therefore, large capacity we should get low potential. 



