TRANSACTIONS OF SECTION A. 605 



half a second, or one second from its initiation. This was a tiling that might be 

 experimented upon. Keferring now to the photographs he observed that there was 

 a great multiplicity of flashes shown in one of them. There were three flashes 

 on one photooraph, and one a great distance at the side — he supposed a mile or so 

 at the side the distance probably would be. Query: Were they simultaneous? 

 Perhaps Mr. Abercromby might say. 



Mr. Abercrombt : We have no evidence ; the photographs were sent to us 

 without any particulars. 



Sir William Thomson, continuing, said that the subject was always a most 

 interesting one, but as it was now coming within the range of experimental inves- 

 tigation (which it could not be before the photographing of flashes of lightning was 

 practised) he thought it would be worth while now to make e.xperiments distinctly 

 to ascertain whether such a group of flashes as that is or is not simultaneous. 



There are some interesting points with regard to the magnetisation of steel 

 needles by a helix in which there are electric oscillations which have been spoken 

 of by Lord Rayleigh. If he remembered right Riess experimented on the subject 

 some time prior to 1854. Riess (if he remembered right — and he was pretty sure 

 he did remember right in this respect) found that when a Leydenjar was dis- 

 charged, through an insulated wire wound up in a helix in the ordinary way steel 

 needles are sometimes magnetised in the direction that would be expected and 

 sometimes in the contrary direction. He believed, as Lord Rayleigh had said, 

 that experimenters have actually found differences of dh-ection of magnetisation in 

 different individuals of a group or bundle of steel wires thus actuated, thus ex- 

 posed to this kind of magnetising action. If they had a very powerful current 

 going in one direction, a current of half that strength going in the opposite direction, 

 a current of a quarter the original strength going in the tirst direction, and so on, 

 then they might expect the medium to be left magnetised by the current going in 

 that one direction extending to the last time it had magnetising force enough to 

 reverse the magnetism of the needle. So that he thought by experimenting upon 

 Riess' old experiment with the same degree of electric magnetic inertia in different 

 successive cases but different degrees of resistance, and therefore different rates of 

 extinction of the oscillation, probably full explanation and investigation of them 

 might be made, and they would understand how it might be magnetised in one 

 direction and in the opposite direction. He was afraid that he was occupying too 

 much time, but there was just what seemed to him rather an important point with 

 reference to the protection of buildings. It was rather disturbing to find that a 

 lightning-rod has so little protecting power as Professor Lodge had pointed out to 

 have been proved by experiments. With reference to this distinction between iron 

 and copper he would like to ask Mr. Preece whether he had experimental evidence 

 of any superiority of copper. It was an exceedingly ditflcult question, but Mr. 

 Preece had told them that his experience generally is that lightning-conductors 

 are efficacious whether they are copper or iron conductors. There are certainly a 

 great many iron conductors and he (Sir William Thomson) did not know of any 

 experiment which proved that they were less efficient. On that he could not say 

 anything certain at the present time. As Professor Lodge himself had pointed 

 out, experiment after experiment must be gone through before they could say which 

 it would be safer to recommend for lightning, whether iron or copper. There was 

 one point upon which iron has greatly the advantage, that is, that it takes a great 

 deal more heat to melt it. If they were to compare the cost of iron and copper 

 they would have four or five times as great a mass of iron for a given sum. Take 

 also the higher melting-point of iron and then they woidd see that for the same 

 expense upon a lightning-conductor they would allow for the consumption of a 

 great deal more energy within itself without destroying it by using iron than by 

 using copper ; but then the question of self-induction bearing on magnetisation 

 must be considered before they could say for certain that in aU circumstances an 

 iron lightning-conductor is as safe as a copper one. He thought that the one 

 moral, the one conclusion, which could be drawn from all this was, that a sheet- 

 iron house with sheet-iron roof and sheet-iron walls and sheet-iron floor is the very 

 safest place that we can possibly be in, or that gunpowder can possibly be in, in a 



