81 
of Edinburgh, Session 1875-76. 
bably not exactly that of the wire of measured resistance. The 
error, however, must have been very slight. 
The course of procedure was as follows : — The wire was heated 
red hot in an alcohol flame. After cooling, its specific gravity was 
determined. It was then fastened by the copper plates to the 
thick copper wires, and a slight weight was attached just sufficient 
to straighten the wire, that its length might be accurately ascer- 
tained (it was straightened as much as possible by being drawn 
between the fingers before being fastened in the apparatus). When 
straight enough for the determination of its length, its resistance 
was also measured by the method of double observation, as de- 
scribed in the u Galvanismus ” (see above).* After the determina- 
tion of resistance, weights were carefully piled upon the carrier, 
which during the operation was held fast from below. They were 
then allowed to stretch the wire gradually until it hung quite free, 
and its elongation had ceased. Then the length, the resistance, 
and finally the length a second time, were determined, the second 
measurement of length being made in order to be certain that there 
had been no further elongation. The weights were now carefully 
taken off, the carrier being again supported from below, and the 
specific gravity of the wire was measured as before, the compressed 
ends, however, having been cut off. A large number of experiments 
were rendered useless by the fact that too great weights were attached. 
Either the wires broke, or they were found on inspection to have 
too variable a diameter to be regarded as uniform. In the result 
given below such small weights were used that almost no difference 
of diameter throughout the whole length of the wire could be noticed 
by means of a magnifying instrument. Thus the wire could be 
treated as uniform, and the specific gravity method assumed to 
give its diameter. 
The results of the examination of three wires are given in the 
following table: — 
* Instead of the formula given in u Galvanismus,” the following was used: — 
s*+ - d 2 ) + s(d 1 d 12 + d%d 12 - Zd x d^~ d^d^d^ — 0. 
The length of the German-silver wire as found by this formula was 1108795 
mm. As measured by the cathetometer its length was 1108:8 mm. 
