ON STANDARDS OF ELECTRICAL RESISTANCE. 143 
to some slight extent, especially when great care is not used, and when the 
wires are drawn by different persons. This may explain why, with some metals 
and alloys of the same preparation, conducting powers are often obtained 
which vary several per cent. For instance, W. Thomson* found the conduct- 
ing power of several alloys of copper which he had had made and tested to 
alter considerably on being drawn finer; some of them were faulty from the 
cause we have just mentioned, and, on their being drawn finer, these places 
showed themselves, and were then cut away. 
It has also been showny that when copper wire is heated to 100° for seve- 
ral days, a permanent alteration takes place in its conducting power: thus, 
with the first wire experimented on, it increased almost to the same extent 
as if it had been annealed. With the second wire the increment was not 
nearly so large as with the first, and with the third it hardly altered at all. 
That this is not due to one or the other of the wires being faulty in the just- 
mentioned manner is proved, 
1st, By the close agreement in the conducting powers. 
2nd, By the close agreement between the differences in the values found 
for the conducting powers of the hard-drawn and annealed wires. They 
were— 
1st wire 2nd wire 3rd wire 
at 0°. at 0°. at 0°. 
Hard-drawn ........ 99-5 100-0 100°3 
mmealed ss... voy. 101°8 102-1 102:2 
The values given for the hard-drawn wires are those which were observed 
before the wire was heated at all. 
3rd, That the same occurs with pressed wires: thus, with bismuth it was 
found that the pieces of the same wire behaved differently ; wire 1 showing, 
after 1 day’s heating to-100°, an increment in the conducting power of 16 
per cent., whereas wire 2 increased, although a piece from the same length of 
wire, 9 per cent. 
Again, take the case of tellurium, and taking the conducting power of each 
bar at first equal to 100, we find that the conducting power of bar 1 had 
decreased after 13 days’ heating to 4, where it then remained constant, that 
of bar 2 after 32 days became constant at 19, and that of bar 3 after 33 days 
at 6. 
The cause of these marked changes in the conducting power must therefore 
be looked for in the molecular arrangement of the wires or bars employed. In 
the case of copper, they may be, and probably are, due to the partial annealing 
of the wires ; for we find that wire 1, although the conducting power increased 
after having been kept at 100° for several days almost to the same extent as 
if it had been annealed, yet, on annealing it, it only gained as follows (the 
results obtained with wires 2 and 3 are added) :— 
1st wire 2nd wire 3rd wire 
at 0°. at 0°. at 0°. 
Hard-drawn ............ 99°5 100-0 100°3 
After being kept several j - ‘ 
days at 100° re RM RW Tsk sy cl ene pe 
After annealing.......... 101'8 102-1 102-2 
The above shows that, in all probability, the annealing plays here a part, 
but not the whole, in the change ; for otherwise why do the wires behave dif- 
* Proceedings of the Royal Society, vol. xi. p. 126. t Phil. Trans. 1862, pt. 1, 
