718 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
bar, or by putting it once more on its wooden core ; and it was tested in the galvano- 
meter circuit with the application of glass heaters as before. Not the slightest trace 
of a current was now found ; a result verifying the conclusion arrived at by Magnus, 
that it is not peculiarities of form in different parts of a circuit of one uncrystallized 
metal, but variations in its quality as to mechanical strain, that can ever give it con- 
tinuous thermo-electric action. 
122. It has thus been proved that a circuit of iron permanently strained by pressure 
across the lines of conduction acquires the same kind of thermo-electric quality as 
that which Magnus first discovered to be produced by the lateral pressure com- 
pounded with longitudinal traction, which the process of wire-drawing calls into 
play, or as that which I had myself found to result from a simple traction, leaving a 
permanent elongation after the force is removed. In all these cases the iron is found 
to be harder than it was before acquiring the strain, or than it becomes again after 
being annealed. Hence the nature of the thermo-electric effect in each of the three 
cases falls under the designation “ current from hard to soft through hot,'" by which 
Magnus stated his result as regards iron. This is just as is to be expected from the 
crystalline theory; since longitudinal extension has a common characteristic with 
lateral condensation in the theory of strains, and only differs from condensation 
uniform in all transverse directions, by a certain degree of absolute dilatation which 
accompanies it, instead of the slight absolute condensation accompanying the lateral 
condensation as an effect of pressure all round the sides. In fact the agreement 
between the charaeters of the thermo-electric effects due to longitudinal traction 
and lateral pressure, and again between the reverse characters of the effects of per- 
manent longitudinal extension and those of permanent lateral compression established 
by the experiments which have been described, proves that these effects are due to 
distorting stress, and to permanent distortion, in the main, and leaves it quite an 
open question, only to be decided by further experimental investigation, what may 
be the effects of uniform pressure and of permanent uniform condensations or dila- 
tations. 
123. The crystalline theory is really unavoidable when it is thus established that 
the effect discovered is due to distortion; but still, as the one designation “current 
from hard to soft through hot” applies to all the cases of permanent strain in iron as 
yet experimented on, I thought it necessary, for removing the possibility of objec- 
tions, that an iron conductor giving a current from soft to hard through hot, should 
be constructed. I therefore took twenty-four small soft iron bars turned in a lathe 
to a cylindrical form Jth of an inch diameter, and each an inch long, with flat ends ; 
and compressed twelve of them longitudinally in a Bramah’s press, so as to perma- 
nently shorten each by about ^th of an inch. They were then set in a wooden board 
cut to hold them firmly lengthwise in two rows, those hardened by compression and 
those left soft, being placed alternately with their ends in contact. The end pieces 
towards one side were connected with one another by a little slip of iron touching 
