AND STRAIN ON THE ACTION OF PHYSICAL FORCES. 
171 
21. The change of density which can be effected in metals by permanent extension, 
hammering, or torsion is small. 
22. The amount of recovery of electrical conductibility which is produced by time 
in all metals which are in a state of strain varies considerably with the nature of the 
metal; with platinum-silver the amount of recovery in a given time is very small, and 
with German-silver comparatively very large. 
23. The recovery of electrical conductivity is in all cases attended with increase of 
longitudinal and torsional elasticity. 
24. Metals may be divided into two classes, as far as the influence of permanent 
strain on the susceptibility to temporary change of resistance from change of tem¬ 
perature is concerned. In one class the strained wire is most increased in resistance 
by rise of temperature up to a certain limit, whilst beyond this limit further strain 
diminishes the first effect. In the other class the converse takes place. 
25. There is a close relationship between the thermo-electric properties of strained 
and unstrained metals and their susceptibility to change of resistance from change 
of temperature. 
26. The elasticity of annealed iron or steel is not temporarily but permanently 
increased by raising the temperature of these metals to 100° C. The ductility of 
annealed iron may also be very considerably and permanently diminished by the same 
process. Mechanical strain influences the elasticity in the same manner as the strain 
caused by tempering, and we may say of both kinds of strain that in the case of iron 
and steel there are three “critical points”—very slight strain increasing, moderate 
strain diminishing, and excessive strain again increasing both the- torsional and the 
longitudinal elasticity. 
27. The temporary alteration of elasticity which is effected in the case of nickel by 
raising the temperature to 100° C. is very noticeable. Still more remarkable is the 
temporary alteration of susceptibility to change of resistance from change of stress 
which is produced by the same means. 
28. The electrical resistances of annealed iron, annealed steel, very hard steel, 
nickel, cobalt, bismuth and zinc are all increased by longitudinal magnetization. 
The alteration of resistance produced by the magnetization of annealed nickel is 
very remarkable. 
29. The amount of increase of resistance produced in iron and nickel by longitudinal 
magnetization depends not only upon the magnetism induced, but also upon the 
magnetizing force, in such a manner that increase of resistance will be produced by 
increasing the magnetizing force, even when the latter does not cause any appreciable 
increase of magnetism. 
O 
30. The increase of resistance which is produced by magnetization is probably not 
merely due to the rotation of the molecules of the magnetized substance as molecules, 
but to the electrical currents, which according to Ampere’s hypothesis are constantly 
circulating round the molecules. 
z 2 
