80 Proceedings of the Royal Society 
compared. For all the observations on a single wire, it had, in all 
cases, as nearly as possible the same temperature. That it might 
not be affected by warm or cold currents of air it was defended by 
a coating of gutta percha, and made to pass through a tube of water 
whose temperature could readily be noted. By dipping into a mercury 
pool it was joined up as a second arm of the Wheatstone’s bridge. 
A length of about 5 mm. of the end which dipped in the mercury was 
well amalgamated. Above that the wire was varnished by a non- 
conductor, so that contact began always at the same point of the wire. 
The other two arms of the bridge consisted of the segments of a Ger- 
man-silver wire, — Kirchhoff’s form of the Wheatstone bridge being 
used exactly as described by Wiedemann in his “Galvanismus.”* 
The galvanometer used was Wiedemanns mirror galvanometer, f 
the deflections of the mirror being observed by means of a telescope. 
The current employed was that of a Bunsen’s cell of great internal 
resistance. The length of the wire was determined by a very delicate 
cathetometer, which could measure accurately to *02 mm. The 
lower end of the copper wire which was fastened to the beam, was 
smooth and flat, and cut at right angles to its vertical axis. The 
edge of the small plate was correspondingly cut, so that the exact 
point at which the silver wire was seized and compressed by the 
copper plate could be seen through the telescope of the catheto- 
meter. The clamp which seized the lower end of the silver wire 
was arranged in the same way. The wires, of whose resistance 
measurements were made, were of pure silver, and were carefully 
drawn by M. E. Stohrer, philosophical instrument maker of Leipzig. 
They were always raised to a red heat before being subjected to 
tension, care being taken that fusion did not occur in any part. In 
order to determine the effect of tension on the conductivity of the 
wires, it was necessary to know the relation of their diameter before 
to their diameter after being stretched. This was estimated by a 
careful measurement of lengths and specific gravities. For the latter 
purpose a chemical balance was employed, which could weigh accu- 
rately to ’0001 grm. As the wires had to be rolled up to prevent 
their touching the sides of the vessel containing the distilled water 
in which they were weighed, the measured specific gravity was pro- 
* “ Galvanismus,” vol. i. pp. 251-255, 2d German ed., 1872. 
t “ Galvanismus,” voL ii. pt. 1, pp. 227-230, 2d Ger. ed., 1873. 
