720 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
order of position the twelve cells with the junctions which they surround. A deflection 
of the galvanometer needle, amounting to about 4°, was now observed ; and when the 
cold water and steam supplies were interchanged in the two sets of tubes, an equal 
reverse deflection almost immediately took place. The current indicated was always 
in many trials from uncompressed to compressed through hot in the iron of the circuit. 
125. Here then we have a case of thermo-electric action in iron giving a current 
fr'om soft to hard through hot ; not as found before, “ from hard to soft through hot.” 
Hence it is not pieces of hardened iron in general, but the direction of extension or 
directions perpendicular to the direction of compression, in iron hardened by extension 
or by compression, that have the thermo-electric quality of deviating from soft iron 
towards bismuth ; and a line of compression, or (as we may now safely conclude) 
lines perpendicular to a line of extension, have the reverse deviation, that is deviate 
from soft iron towards antimony, in the thermo-electric series. [^Addition, Dec. 
1856. — Subsequently to the reading of the paper, I have, in verification of this con- 
clusion, found, by a direct experiment, that a conductor of sheet iron, hardened by 
lateral extension and softened in parts, has the thermo-electric property of giving a 
current from soft to hard through hoti] The crystalline theory being thus fully 
established for the thermo-electric effects of mechanical strain in iron, whether tem- 
porarily induced during the application of stress, or remaining with molecular dis- 
placement after the stress is removed, we may readily suppose it will be found to 
hold equally for all thermo-electric effects any metal can experience from mechanical 
action, except the hitherto undiscovered effects of condensations or dilatations equal 
in all directions. The experiments I have already made on other metals than iron, 
do not go further in verifying the crystalline theory than to show for copper and tin 
wires what I had previously shown for iron, that the same thermo-electric effect in a 
linear conductor is produced by permanent longitudinal extension and permanent 
lateral compression. 
126. The process of raising to a high temperature and then cooling very suddenly, 
produces a marked effect on the mechanical qualities of most metals, especially on 
their hardness ; and generally all that is neeessary to do away with this effect and 
restore the metal to its primitive condition, is to keep it for some time at a high 
temperature and let it cool slowly. This process being called annealing, I shall for 
brevity designate as unannealed, any substance which has been subjected to the former 
process (sudden cooling) and which has not been subsequently annealed. It is not 
easy to judge exactly of the relation of the strains in the different parts of an unan- 
nealed piece of metal, to simple mechanical strains; but some thermo-electric effect, 
whatever its exact nature and explanation may be, is to be anticipated, with so great 
a change of other qualities as many metals experience in the process of sudden 
cooling ; and it may be readily supposed that different thermo-electric qualities will 
be found in unannealed pieces of different shapes. I have therefore made experiments 
on the thermo-electric differences between unannealed and annealed linear conductors 
