158 
MR, H. TOMLINSON ON THE INFLUENCE OF STRESS 
receiving fresh magnetism is lessened in the armatures by previous powerful magnetiza¬ 
tion this induced current would be lessened , and thus, if their mode of experimenting 
was adopted, the resistance of the wire would seem to be decreased. The following 
experiment was tried with a view of testing the effect of previous magnetization 
on the capability of receiving fresh magnetism in the case of the armature of an 
electromagnet. 
Experiment LXXIV. 
A bar of soft iron, 12 inches long \ inch broad and b inch thick, was placed across 
the cores of a small electromagnet, and separated from them by a piece of tissue paper. 
The cores of the electromagnet were 1 inch in diameter and 5 inches in length, and 
were wound round with the cotton-covered copper wire yg-th of an inch in diameter, 
and having a resistance of nearly 1 ohm. A battery of six Grove’s cells was employed 
with the electromagnet, and in order to test the change of susceptibility to magnetiza¬ 
tion, some 200 turns of rather fine silk-covered copper wire were made round one end 
of the bar of soft iron and distributed over a length of 4 inches. The coil thus formed 
was connected with the galvanometer, and was further insulated from the iron core by 
two layers of paper. The coil was then placed inside the coil B, which itself could be 
placed in the circuit of one large Daniell’s cell by means of a mercury-cup. The 
current induced by the magnetization imparted by the coil B to the soft iron bar 
was measured by the “ throw ” of the galvanometer-needle produced when the circuit 
of the Daniell’s cell was closed, first without exciting the electromagnet and then 
with this magnet in action. The electromagnet was at such a distance from the 
galvanometer that no error of importance caused by the direct action of the former 
would be introduced. The following experiments were then made :— 
