PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 713 
of boiling water. If the wire be well annealed at the commencement of the experi- 
mentj and if weights be gradually added to the lower side of the frame, the galva- 
nometer needle gradually moves to one side, indicating a current from the unstretched 
to the stretched round the hot peg ; and the deflection goes on increasing as long 
as weights are added, up to the breaking of the wire. If, however, before the wire 
breaks, the weights are gradually removed, the needle comes back towards its zero- 
point, reaches zero, and remains there when a certain part of the weight is kept 
suspended. If this is removed the needle immediately goes to the other side of zero, 
and remains, indicating a current from the strained part into the unstrained part of 
the iron wire round the part wrapped on the hot peg ; that is, from strained to 
unstrained through hot, or as Magnus found, “from hard to soft through hot.” 
111. If weights be added again, as at first, this deflection is done away with, and 
the deflection that first appeared is regained, when the weight which previously 
allowed the needle to return to zero is exceeded. We thus conclude that iron wire 
hardened by longitudinal tension, may, by the application of a certain longitudinal 
force, have its thermo-electric quality reduced to that of unstrained soft iron, and by 
a greater force may be made to deviate in the other direction ; or that hardened iron 
under a heavy stress, of the hind by whieh it has been hardened, and hardened iron left 
free from stress, are on different sides of unstrained soft iron in the thermo-electrie series. 
There can be no doubt but that the same property holds for copper wire, being in 
fact demonstrated by the experimental results described above in §§ IO 7 and 109. 
112. I have not yet investigated the thermo-electric effects of stress (that is, the 
effects accompanying temporary strain) in other metals than iron and copper ; but 
it appears probable that the same law of relation to the thermo-electric effects of 
permanent strain without stress will be found to hold in each case, since it has been 
established for two metals in which the absolute thermo-electric effects are of con- 
trary kinds. I hope, however, before long to be able to adduce experimental evidence 
which will supersede conjectures on the subject. [Since this paper was read I have 
verified the same law for platinum wire.] 
113. The object which was proposed in entering on the investigation, being to test 
the thermo-electric properties of a strained metal, in different directions with reference 
to the direction of the strain, was not attained by comparing the thermo-electric pro- 
perties of a longitudinally strained metal with those of the same metal in its natural 
state; but it would certainly be promoted by discovering the effect of lateral pressure 
on a wire in modifying its longitudinal thermo-electric action. I therefore made the 
following experiments on the thermo-electric effects experienced during the applica- 
tion of a moderate lateral pressure, and of permanent strain after the cessation of 
excessive lateral pressure, in various wires. 
114. Experiment to discover the temporary effect of lateral pressure on the thermo- 
electric quality of iron wire: — A rectangular bar of iron (If inch square), with pieces 
of thin hard wood placed on two opposite sides, had fine iron wire laid in a coil of about 
twenty turns round it. The wood perfectly insulated the wire from the iron bar, and 
