AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
97 
The wires were now removed from the air-chamber and the one which had been 
stretched was remeasured. The same processes were repeated after greater and 
greater loads until finally the wire was broken. With some of the wires the specific 
gravity was determined after each stretching, but as the alteration of density was 
found to be very small with most of the substances, the specific gravity was deter¬ 
mined before the stretching had commenced, and again after the wire had been 
considerably elongated; the density for intermediate amounts of stretching was 
calculated on the assumption that the change of density is proportional to the increase 
of length. 
In the second method the wires were kept during the whole period of observation 
in the air-chamber ; the hook on the pulley was connected by a fine copper wire, with 
a brass sliding piece provided with an index and capable of free vertical motion up and 
down a wooden scale. The lower extremity of the sliding piece was slightly weighted, 
so as to keep the fine copper wire, which had been previously stretched very nearly to 
breaking, perfectly straight. The hook on the pulley was also connected by a chain 
with a lever which served to elongate the wire. On commencing the experiments the 
weight of the lever was removed from the wire and the resistance of the latter was 
determined ; the position of the index was then noted and afterwards the lever was 
used to produce the required extension, when again the wire was relieved from stress 
and after a few minutes the resistance and length of the wire were redetermined. In 
this way an alteration of length not exceeding gwoo-th of the whole could be readily 
measured, but as it was ascertained that in the case of certain wires it was necessary 
to remove entirely even the slightest constraint, such for instance as would be caused by 
the small weight of the pulley or of the sliding piece, before they would attain in a 
sufficiently short time their ultimate resistance, a third method was adopted as 
follows :— 
The length of the wire to be examined having been measured as in the first method, 
it was placed, together with the comparison-wire, at full length in a long wooden box 
made for the purpose, the two wires having previously been wrapped as usual in paper 
or surrounded by caoutchouc tubing, and after a sufficient time had elapsed, usually 
about 15 minutes, to enable them to assume their ultimate ratio of resistance, this 
latter was determined. The wires were then removed from the box, and the one to be 
tested stretched as in the first method to a certain extent; they were then replaced, 
and after the proper time their resistance ratio redetermined. At each removal of the 
wires from the box the connexions with the other parts of the bridge had to be 
disturbed; but it was ascertained, as indeed might have been expected from the mode 
of experimenting,* that this did not in the least affect the value of the ratio of the 
resistances of the two wires. A few examples will suffice to show the nature of the 
results obtained. 
* I have frequently removed and replaced the connexions of the wires without causing any alteration 
of resistance which would amount to T ohoo^ the whole. 
MDCCCLXXXIIT. 
O 
