AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
9 
Remarks on the preceding experiments. 
It appears from the last table that the temporary elongation produced in an iron 
wire by a load of given magnitude becomes greater as the permanent elongation 
becomes greater up to a certain limit of the latter, which limit seems to depend upon 
the load used to produce the temporary effect. When the above-mentioned limit has 
been reached further permanent elongation begins to increase the elasticity, and this 
increase, as other experiments have shown, is continued up to the breaking point of 
the wire. 
The increase of elasticity produced by rest, which is very conspicuous when we 
compare III. and IV., is the more remarkable as it is not attended, as was at first 
supposed would be the case, by any appreciable permanent shortening; the latter 
amounting in the present instance to only '1 mi H im , out of a length of 8600, actually 
not ‘002 per cent. 
This phenomenon is moreover evidently closely allied with one noted by Bottomley,'"' 
who has recently discovered that in the case of iron the permanent elongation which 
can be produced by any weight may be very largely diminished by putting on this 
weight in small quantities at a time with intervals of rest between, and also that the 
breaking stress may be considerably increased by the same process. 
We may assume that the mutual attraction existing between the molecules of a 
wire will always tend to make them take up such positions as will give a maximum 
mutual attractive force. When, therefore, a wire has been permanently stretched, the 
molecules would immediately take up these positions were it not for coercive force; 
this, however, causes delay, so that if the wire were tested shortly after the permanent 
extension has taken place, the elasticity would be found to be less than when the mole¬ 
cules have had sufficient time to finally settle themselves, Nor is it necessary that any 
appreciable permanent contraction of the wire should attend this increase of elasticity, 
as the mere change of'arrangement of the molecules would suffice for the purpose. 
To a similar cause is no doubt also due the increase of portative power which can be 
produced in a permanent magnet by gradually increasing its load. 
Another point to be noted is the sub-permanent set which is produced in all wires ; 
this set is greater, according as the load permanently left on the wire is greater, and 
also increases up to a certain limit with the time during which the stress producing 
the set is applied : from this it follows, that the readings taken on loading a wire step 
by step to a certain amount will be different from those taken at the same stages of 
stress on unloading. This fact has already, I believe, been noticed by Thomson ; but 
as I wish particularly to draw attention to it in the case of iron, I give the results of 
one out of several experiments made on iron and copper with a view of testing the 
matter. 
* Proc. Roy. Soc., vol. xxix., p. 221. 
MDCCCLXXXIII. 
C 
