STRAIN ON THE PHYSICAL PROPERTIES OF MATTER, 
827 
by means of hooks from the bar, and afterwards loaded carefully with weights, which 
eventually reached 16 kilos. 
Load added to the 
bar in grammes. 
Untwist in scale- 
divisions produced by 
each additional load. | 
388-8* 
1814-0 
1000 
453-5 
1000 
113-8 
2000 
106-3 
2000 
78-3 
2000 
59-3 
2000 
43-8 
2000 
45-8 
2000 
39-0 
2000 
37-8 
The Effect of Permanent Strain on the Internal Friction. 
Experiment XVI. 
An annealed iron wire. 602 centimetres in length and 0'960 centimetre in diameter, 
' O 
was provided with a vibrator whose mass was 3606 grammes; a rest of 24 hours was 
now allowed, and after this period the wire was tested, the results of the experiments 
showing a logarithmic decrement of •001131. Next the wire was permanently 
elongated to the extent of 3*16 per cent., and, having been shortened to its original 
length, was again tested about ten minutes afterwards ; the logarithmic decrement 
was now found to be '001422. After a rest of 24 hours, the internal friction had so 
far diminished that the logarithmic decrement had become '001214, i.e., nearly 8 per 
cent, greater than it was before the permanent extension had taken place. The vibra¬ 
tion-period before the permanent strain was 4'309 seconds, and after the extension 
4'505 seconds ; therefore, allowing for the diminution of section, resulting from the 
strain, the effect of the latter was to cause a decrease of the torsional rigidity of 2'8 
per cent. 
We must observe, with reference to such an experiment as the above, that, in order 
that permanent extension may produce increase of internal friction, the wire must be 
loaded, when tested before the extension, to a degree which, though not sufficient to 
cause sensible permanent elongation, is yet enough to secure a certain amount of 
permanent strain among the molecules; if the wire be only tested with a light 
vibrator, permanent extension may possibly produce a diminution of internal friction, 
as the following experiment shows. 
* The load produced by the pan itself. 
5 N 2 
