712 PROFESSOR THOMSON ON THE ELECTRO-DYNAMIC QUALITIES OF METALS. 
by a longitudinal force bears to copper wire of the same substance unstretched, the 
same thermo-electric relation as that of bismuth to antimony. 
108. I next made a similar experiment on iron wire, varying the arrangement so that 
the weights could be rapidly shifted ; and again so that equal sets of forces could be 
applied to one or to the other of the two sets of wires, merely by pressing with the foot 
upon one or anothei- of two levers. A perfectly decided result was at once obtained ; 
and I ascertained that the thermo-electric effect was induced and lost quite suddenly 
on the pressure being applied and removed. In this case the nature of the effect was 
the reverse of that found in the experiment on copper, the deflections being always 
such as to indicate a current in the iron wires from unstretched to stretched through 
the hot junctions. 
109. The thermo-electric effect which these experiments demonstrated to accom- 
pany temporary strain produced by a longitudinal force, was, in each of the metals, 
the reverse of that which Magnus * had previously discovered in the same metal 
hardened by the process of wire-drawing, and which I ascertained for myself to be 
produced in each case when the metal is hardened by simple longitudinal stress 
without any of the lateral action inseparable from the use of the draw plate. I thus 
arrived at the remarkable conclusion, that when a permanent elongation is left after 
the withdrawal of a longitudinal force which has been applied to an iron or copper 
wire, the residual thermo-electric effect is the reverse of the thermo-electric effect 
which is induced by the force, and which subsists as long as the force acts. 
110. I have made a single experiment demonstrating this conclusion for iron by 
means of a multiple tension apparatus, similar in principle to that described above 
(§ 105). But with a somewhat more sensitive galvanometer than the one I used, the 
result may be shown in a perfectly decided manner (for iron at least) without any 
multiplication of the thermo-electric elements ; and a very striking experiment may be 
made on the following plan: — A thin iron wire is wrapped three or four times round 
a wooden peg held firmly in a horizontal position, and again two or three times 
round another parallel peg, about 4 inches lower. A 
frame is rigidly connected to this second peg, so that 
it may remain stably in a horizontal position ; hang- 
ing from the wire and pulled down by the frame with 
either a light or a heavy weight attached to its lowest 
point. To keep the wire from slipping, the parts of it 
running from the pegs towards the ends are kept 
stretched by light weights tied to them ; and the slack 
parts below these weights are carried away to the 
galvanometer electrodes, with which they are connected in the manner described 
above (§ 92). Any convenient source of heat is applied to the part of the wire bent 
round either peg, so as to keep it at some temperature, perhaps about as high as that 
* Poggendorff’s ‘Annalen,’ Aug. 1851. 
