598 REPORT— 1888. 



the coats of a Lejden jar wliicli are preparing to spark into one anotlier and which 

 do spark into one another when the difference of potential rises to a certain 

 maximum and the area is broken down. 



Concerning the duration of flashes, that again was a point on which much 

 Tahiable work might be done by meteorologists and photographers. He had seen 

 lightning flashes which certainly appeared to last two or three seconds. He could 

 not imagine that it was one flash which was doing that ; he thought it was a series 

 of multiple flashes succeeding one another very rapidly. But whether they lasted 

 long or not there was no necessary argument that they were not oscillatory. The 

 fact that they deflect a compass needle does not prove anj-thing about it, and does 

 not prove anything concerning their duration either because a ballistic galvanometer 

 is deflected by a momentary kick — a momentary blow given to a thing can deflect 

 it, the blow having ceased long before the motion has ceased which it produced. 

 But there is a difficulty which Mr. Preece did very well to advert to, and that is 

 the magnetising power of a lightning flash. He thought that was the strongest 

 point Mr. Preece had adduced. A flash magnetises steel bars, deranges the mag- 

 netism of a ship's compass, and conspicuously produces magnetic effects. Now an 

 oscillating current ought not to produce these effects. An oscillating current with 

 decaying amplitude is used by Professor Ewing to demagnetise steel, not to mag- 

 netise it. It ought not to magnetise, but it does magnetise them ; therefore how 

 can it be a current of this kind ? But then the same difficulty would be felt with 

 a Leyden jar discliarge. The Leyden jar discharge is also oscillatory, certainly 

 oscillatory because it has been seen to be spiral ; the sparks have been analysed iii 

 a revolving mirror, and yet it magnetises steel needles when sent round them. He 

 did not understand that point himself, but he hoped that I.ord Rayleigh might say 

 something about it. Mr. Preece was quite riglit in saying that the whole theory 

 depends upon the oscillations. What he (Professor Lodge) had done, as far as tlie 

 theory was concerned, was merely to call attention to these oscillations (which were 

 •well known in the case of Leyden jar discharges and of which the mathematics had 

 been worked out) and to point out that they applied also to lightning which he 

 thought had not been much noticed. 



What the energy of a flash is he did not know. He wished there were some 

 means of determining it ; it would be very important to determine the energy of a 

 flash. He would now say a word as to whether lightning was likely to be oscilla- 

 tory or not — this question of what the resistance or friction has to do with tlie 

 energy of the capacity which is discharged. The smaller tlie capacity the more 

 likely it is to be oscillator}' — the bigger the electro-magnetic inertia the more likely 

 it is to be oscillatory. Now the capacity discharged in a flash is small. He said 

 it was small for this reason, that the quantity of electricity discharged is not great. 

 It was well known that the quantity of electricity concerned in a lightning flash 

 was small. The quantity of electricity then existing in the portion of the cloud 

 that is discharged is a small quantity, but the potential of it is enormous. It is 

 able to spark .500 feet, possibly a mile — whatever it is able to spark it does spark. 

 That meant an enormous potential. Now when they had a very small quantity 

 raised to an enormous potential that meant that the capacity could not be great, 

 capacity being the ratio of the two. There was no reason for supposing that the 

 capacity of a condenser discharge in a lightning flash is anything bigger than a 

 Leyden jar's, or in a micro-Farad, or things of that sort. He would mention as 

 being perhaps interesting that the radiation of the waves produced by a micro- 

 Farad condenser discliarging from a coil of self-induction one secohm, as Professors 

 Ayrton and Perry called it — he preferred to call it a 'quod' — the waves produced by 

 that discharge— the ether waves, waves of light— if it is oscillatory will be 1,200 

 metres long. They had nothing like that capacity discharging in a Leyden jar, 

 which has a capacity only, as he guessed it, of 10 metres electro-static units. 

 Mr. Preece had promised to supply him with covered wire in order to repeat these 

 experiments on the alternative path. He would have the greatest pleasure in taking 

 advantage of it. 



Then there was the theory of protection of area. Mr. Preece said, ' If it does 

 not protect the area what is the good of it ? ' Well, but that was not an argument 



