of Edinburgh, Session 1875-76. 
79 
Monday, 3 d January 1876. 
Sir WILLIAM THOMSON, President, in the Chair. 
The following Communications were read : — 
1. On the Electrical Conductivity of Stretched Silver Wires. 
By J. G. MacGregor, M.A., B.Sc. Communicated by 
Professor Tait. 
The apparatus which I used in a few experiments on silver wires 
was as follows : — To a beam, supported in stonework, a plate of 
copper was fastened, upon which a smaller plate could be tightly 
screwed. Between the two plates a very thick copper wire was 
secured, vertically. Its lower end was provided with a small plate 
of copper, fastened by screws. This plate served to make fast one 
end of the silver wire under investigation. The other end was 
joined in the same way to a second thick copper wire; this was 
provided with a horizontal round brass plate, through the centre of 
which it passed, and which acted as weight-carrier. A length of 
about 8 mm. at the end of the part of the copper wire which projected 
below the weight-carrier was amalgamated, and, while hanging 
quite free, dipped into a glass cup containing mercury, which, by 
means of a long screw, could be elevated or depressed by any desired 
amount. When measurements of resistance were made it was 
always placed in such a position that the amalgamated part of the 
copper wire was just beneath the surface of the mercury. The 
glass cup served also to support the weight-carrier during the 
adjustment of the weights, that the silver wires might be sub- 
jected to no jerks. After putting on weights the cup was lowered 
very slowly and steadily until the weights hung free. A copper 
wire (4 7 mm. thick and 30 cm. long), dipping in the mercury, 
joined up the silver wire as one of the arms of a Wheatstone’s 
bridge. At the upper end of the copper wire, which was fastened 
to the beam, two other copper wires were fastened by binding 
screws. One of them went to the galvanometer; the other was the 
standard wire, with whose resistance that of the silver wires was 
