158 
Proceedings of Royal Society of Edinburgh. [sess. 
instantly perceived a painful sensation of burning, and let the bar 
drop. He found his hand or fingers, where it had touched the bar, 
severely blistered. The bar itself was found immediately after- 
wards to be quite cold, or not perceptibly hot. This was a very 
marvellous incident. It proved (1) the outer surface of the steel 
to have been intensely heated ; (2) that not enough of heat was 
generated to sensibly warm the whole bar. The explanation, of 
course, was to be found in the known laws of diffusion of electric 
currents, through non-magnetic conductors, considered in connection 
with the effect of magnetic susceptibility of unknown amount and 
law, in conductors of steel or iron. 
Lord Armstrong’s accidental experiment seemed to me such a 
very instructive illustration of fundamental principles of electro- 
magnetic induction, that I wrote to him asking his permission to 
communicate it to the Eoyal Society of Edinburgh, at the same time 
inquiring as to some details. In reply I immediately received a 
letter, of date 7 th March, kindly giving the desired permission, and 
containing the following very interesting statement : — 
“ I send you, by parcel post, the steel (not iron) bar which I held 
££ when it accidentally short-circuited the current. You will observe 
“ two little hollows,* which were burnt out of the metal at the 
“ instant of contact, and these mark the distance between the points 
“ of contact. The bar was held by my fingers midway between 
u these two marks, and the bums were inflicted at the places where 
“ my fingers touched the metal. The sudden pain caused me to 
“ dash the bar instantaneously to the ground, and an attendant 
“ immediately picked it up and found it quite cold. Three of my 
“ fingers and my thumb were blistered, and had the injuries not 
“ been immediately treated by an expert who happened to be 
“ present, they would probably have developed into troublesome 
“ sores ; as it was, my arm had to be carried in a sling during the 
tc first day, and I was not able to hold a pen with comfort for many 
“ days afterwards. There was a great blaze of light from the two 
“ points of metallic contact, but the flame could not possibly have 
“ got under my fingers where they touched the metal and were 
££ burnt. If the flame had done the injury, it would have taken 
* The distance between the hollows is lS^cms., the bar is about a foot long, 
and its diameter is 14 mm. 
