in changing the dimensions of Iron and Steel bars. 91 
tion contained in the sum of retractions given in the last column 
of the table. Therefore, to obtain the effect of the heat devel- 
oped at the moment of demagnetization, we should subtract ‘03 
from ‘16, the heating effect of demagnetization determined with- 
out this correction. This gives "16—-03='13 of a division; 
while from Table I we deduced ‘12 for the value of the same 
effect. The difference of only ‘01 division in the two results is 
not, however, to be taken without some reserve, for in the cal 
culations I assumed that the rod had the same rate of expansion 
under a closed circuit as under an intermittent one, and this I 
did because I had no means of determining the difference, if 
any exist. Experiments similar to those just given were made 
on all the iron rods, and similar results were obtained. 
Relations existing between the number of break-circuits, the heat- 
ing of the rod, and its elongations. 
At this stage of the investigation, it became of interest to 
h ‘ 
electric couple, formed of two wires, one of copper, the other of 
iron. This compound wire was wrapped, first with two layers 
Circuits, taking care that the closed circuits, preceding the 
break-cireuits, should all be of the same duration. 
_ After series of break-cireuits the elongation produced 
m the rod and the permanent deflection in the galvanometer- 
needles were noted, and the observations showed that the elon- 
gations and the increments of temperature in the rod were pro- 
portional to the number of break-circuits. 
On the elongations and retractions observed in the tron rods as the 
Strength of the magnetizing current is gradually increased and 
diminished, and on the equality in the elongations produced by a 
definite current when it is gradually and when tts suddenly 
ae observed sudden elongations taking place in an Iron rod 
the moment of its magnetization naturally led me to Inquire 
