AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
37 
lead, both the maximum temporary increase of length which can be produced by any 
load, and the complete recovery after the removal of the load, are only attained after 
an interval of several hours, provided that the weights used for the load be not very 
small compared with the breaking-load. 
8. There is a small but decided departure from “Hooke’s law,” as far as temporary 
elongation is concerned, in all cases where the load employed to produce the elongation 
is of moderate amount; this is the case even when sufficient rest has been allowed to 
enable the wire to attain its maximum elasticity. 
9. We can therefore only obtain by the method of longitudinal vibrations values 
for “ Young’s modulus,” which are strictly comparable with those got by the method 
of static extension, by experimenting when we use the latter method with very small 
loads, and with the wire under the same conditions of stress and strain as those 
occurring when the former method is adopted. 
10. In the case of all metals, permanent extension, if not carried beyond a certain 
limit, causes, whether rest is or is not allowed after the permanent extension has taken 
place, a diminution in the value of “Young’s modulus,” as determined by the method 
of static extension. 
11. If the permanent extension be carried beyond the above-mentioned limit, further 
permanent increase of length increases the value of “ Young’s modulus.” 
12. The limit of permanent extension mentioned in 10 varies considerably with 
different metals, and with the time which is allowed to elapse after the permanent 
extension has taken place. 
13. In the cases of iron, heavy loading for some time so increases the value of 
“ Young’s modulus,” as determined by the method of static extension, that even when 
the extension would have caused, without such loading, diminution of the modulus, 
this diminution can be changed to an increase; with copper this is not the case. 
14. With iron wire which has been heavily loaded for some time, the ratio of the 
temporary elongation to the load producing it becomes less as the load employed 
becomes greater, until a certain limit, depending upon the extent of the previous heavy 
loading, has been reached ; whereas with most other metals, and with iron which lias 
suffered permanent extension without allowing the load which has produced the 
extension to remain for some hours on the wire, the elongation increases at first in 
greater proportion than the load. 
15. The effects on the longitudinal elasticity, and on the torsional rigidity of steel, 
of suddenly chilling the metal after it has been raised above a bright red heat, are 
similar to those produced by excessive permanent traction*. 
16. The loss of torsional rigidity, which is caused by twisting or stretching a wire 
beyond the limits of elasticity, is diminished by rest. 
17. The influence of rest mentioned in 16 is greater in proportion for large arcs of 
vibration than for small ones, and is more noticeable with iron and steel than with 
most of the other metals. 
