AND STRAIN ON THE ACTION OE PHYSICAL FORCES. 127 
affected by temporary stress, be related to the susceptibilities of alteration of resistance 
from change of temperature in the same way as the corresponding qualities seem to be 
where permanent strains are concerned, it may be said to remain at present open, and 
only to be decided by further experiments on the effects of stress on the thermo-electric 
qualities of iron;* but as far as copper and nickel are concerned, the above question 
seems to be answered in the affirmative. 
The Effect of Slight Mechanical Strain and of the Strain caused by 
raising Iron to 100° C. and afterwards Cooling, on the Torsional 
Rigidity of the Metal. 
The above experiments had shown such an astonishing influence to be produced on 
the longitudinal elasticity of annealed iron by merely raising the metal to 100° C. and 
then cooling, that it seemed advisable to test whether or not a similar effect would be 
produced on the torsional rigidity of iroiq by a like cause, and if so, whether we can 
imitate the strain resulting from heating and afterwards cooling by mechanical means. 
Experiment LI. 
A piece of annealed iron wire was vibrated at a temperature of 13° C., and the 
time of a single vibration, as determined by counting the vibrations for five minutes, 
was found to be I’OOO second. The wire was then heated in an air chamber to 
100° C., and after having been maintained at this temperature for one hour was 
suffered to cool, and the time of vibration found to be 0'989 second 12 hours after 
cooling. 
Experiment L1I. 
A second piece of the same wire, when suspended ready for vibrating, was heated 
slightly by passing the flame of a Bunsen burner rather quickly up and down it 
several times, the vibrator being at the same time supported, so as to take off stress 
from the wire. The time of vibration before heating was Iff 54 second, and in five 
minutes, 35 minutes, and 245 minutes after cooling was 1*147, 1*142, and 1*136 
second respectively. 
The wire was then heated to redness and cooled, when the time of vibration after 
five minutes was 1*156 second, and after 22 hours became 1*143 second. 
Experiment LIII. 
A third piece of the same wire was vibrated after different slight amounts of 
permanent extension had been produced ; the length before stretching was 82*0 
centims., and the time of vibration 1*621 second. After slightly stretching, so as 
to increase the length to 82*2 centims. and 84*3 centims., the time of vibration 
became respectively 1*614 second and 1*714 second. 
* This point will be fully considered in Part IY. 
