AND STRAIN ON THE ACTION OP PHYSICAL FORCES. 
165 
Experiment LXXIX. 
A piece of annealed iron wire with copper terminals, both of similar dimensions to 
those of the wire and terminals in the last experiment, was treated in the same 
manner as the German-silver, and deflections of 50 and 25 divisions, both on the 
same side, were obtained. The wire and its terminals were in this case well wrapped 
up in paper, and, as in the previous experiment, there was no sensible deflection 
before the Grove’s cell was used. These experiments, which are only two out of 
several which were made with different pairs of metals, show that with the German- 
silver the electromotive force generated by the “ Peltier effect ” was so far greater 
than that due to any other cause that the deflections were nearly the same on both 
sides; whereas with the iron the current produced by the unequal heating of the two 
junctions from other causes than the “ Peltier effect ” predominated, and this was 
found to be the case with several specimens of iron and copper. 
In my experiments of 1875 I balanced iron wires or iron rods against wires of other 
materials, and using rather powerful electromotors (from one to six Grove’s cells) 
proved, as I thought, that the electrical resistance of iron increases with the intensity 
of the current employed in the “ bridge ; ” but in these later investigations, in which, 
having a much more delicate galvanometer, I could obtain a measure of the resistance 
of the substance within 1 in 50,000 with a battery-power one-tenth of the smallest 
then used, I have been unable to detect with certainty any such change. 
As for the discrepancies which exist between my present and former observations on 
both soft iron and hard steel, I can only attribute them to errors caused by the 
magnetizing coil being too close to the iron or steel to allow of sufficient protection 
from errors caused by heat radiated or conducted from the former, and which might 
increase the resistance of the metal as a whole, or cause apparent increase or decrease 
by unequal change of potential at the junctions of the two copper terminals with the 
iron or steel. At any rate, using both steel and iron of the same qualities as used then, 
but adopting more perfect thermal insulation and a more accurate mode of experi¬ 
menting, I have been unable to detect any such considerable increase of resistance in 
the case of soft iron or soft steel, or any decrease of resistance of hard steel, as I did 
then. Now Auerbach, with some of his specimens of iron and steel wires, obtained 
apparent alterations of resistance of 1, 2 and even 3 per cent.—alterations of decrease or 
increase which would have, in the case of specimens of a similar nature used by myself, 
sent the reflected image of the illuminated wire flying off the scale, whereas, instead of 
this, I found nothing but variations of resistance which never, with wires of a similar 
diameter, reached even to T per cent., and this, too, with magnetizing forces which 
must have equalled those employed by Auerbach. 
• Auerbach, again,* seems to concur with Beetz and others that the mere mechanical 
pull connected with magnetizing would have caused an apparent decrease of resistance 
* Loc. cit., p. 151, 
