16 
Proceedings of the Royal Society of Edinburgh. [Sess. 
V. — Note on the Change produced in the Conductivity and 
Density of Lead Wires by Permanent Stretching. By James 
A. Donaldson and Robert Wilson, Natural Philosophy Labora- 
tory, Edinburgh University. Communicated by Professor J. G. 
MacGregor. 
(MS. received February 22, 1907. Read March 4, 1907.) 
The wires used were of good commercial lead and well drawn. 
The special difficulty in experimenting with lead is due to its softness, 
involving initial lack of uniformity in section, which is increased on 
stretching. That the variability of the section of the wire might be 
allowed for in the computation of specific resistance, etc., the diameter 
was measured at the ends, at two points respectively one inch from each 
end, and at eight other points dividing the rest of the length of the 
wire into equal parts. The ends of the wire were soldered into copper 
pieces to prevent serious thinning near the ends when it was stretched. 
The diameter was measured by means of a screw gauge, made by 
Elliot, which reads to '0001 inch. 
In computing the specific resistance two methods were used, both of 
which require that the lack of uniformity between the points of measure- 
ment of the diameter be small. In the first method we considered the 
parts of the wire between the successive points of measurement as forming 
truncated cones, and thus got R == Y -i. where R is the total resistance 
7T 
of the wire, p is the specific resistance, and is the length and r x and r 2 
the radii of the ends, of one of the cones. In the other method we made 
a graph for length along wire against diameter, and thus determined the 
mean diameter, which we used in the formula R = C L ; where l is whole 
7r r l 
length of wire and r is mean radius. 
The arrangement of the apparatus and the method of hanging the wires 
was as follows : — The copper piece, into which the upper end of the lead 
wire was soldered, was clamped to a block of copper containing a mercury 
cup. This block of copper, again, was fixed into a block of wood, which 
was sunk into the wall of the room. To the lower end of the wire to be 
stretched was attached a copper rod, with a disc for weights. This rod 
passed through the disc and dipped into a mercury cup, fixed on a screw 
