128 
MR. H. TOMLINSON ON THE INFLUENCE OF STRESS 
Experiment LIV. 
j 
A piece of annealed pianoforte steel was vibrated after different slight amounts of 
permanent extension. 
Length of wire, 
Time of vibration, 
in centims. 
in seconds. 
89-00 
•984 
89-05 
•976 
89-10 
•968 
89-20 
•968 
These experiments''' speak for themselves, and prove that the torsional rigidity is 
affected in a precisely similar manner to the longitudinal elasticity by raising to 100° C., 
and then cooling, and moreover that the strain produced by slight mechanical traction 
acts in a similar manner on both iron and steel to the strain produced by tempering. 
We see also how very quickly the increase of elasticity is changed into a decrease when 
the extent of strain is widened either by heat or by mechanical means. Evidently 
then there are for iron three critical points as regards its torsional rigidity as 
well as regards its longitudinal elasticity—very slight strain increasing, moderate 
strain decreasing, and excessive strain again increasing both these physical properties. 
Further, it would be interesting to determine whether cold would not produce the 
opposite effect on the elasticity to heat, and it seems highly probable that cooling 
below the temperature of the room will permanently decrease the elasticity of iron ; 
this point, however, I hope to be in a position shortly to decide. 
Further Discussion of Wertheim’s Experiments on Elasticity.! 
We have seen that the temporary alteration of resistance produced by any load is 
permanently decreased in the case of annealed iron wire by merely raising the tempera¬ 
ture of the metal to 100° C. Now Wertheim’s experiments seem at-first sight to 
prove that the elasticity of iron and steel is greater at 100° C. than at the ordinary 
temperature of the room ; but if M. Wertheim had examined the elasticity after the 
wire tested at the higher temperature had again cooled down to the lower one, he 
would have found that this apparent temporary increase of elasticity was really a per¬ 
manent one, and if the wire had been tested several times, first at the higher and then 
at the lower temperature, he would have also found, provided sufficient rest after 
* All these experiments were repeated several times with different specimens of iron, but invariably 
with the same result as regards the nature of the change, 
t Anu. de Chimie et de Phys., 3 me serie, 1844. 
