and form for a compass needle. 125 
hot poker, but from the thickness of the small parallelogram 
this heat was found insufficient, and that of a lamp was em- 
ployed. The needles were then magnetized as in the last ex- 
periment. 
Experiment 35. 
Directive 
force. 
Small parallelogram 
Large parallelogram 
1815 
i6‘6o 
It occurred to me that the heat employed in tempering the 
large parallelogram might not have been sufficient, it was 
therefore exposed to the flame of the lamp, but in doing this, 
a small piece which weighed 10 grains was broken off from 
its end. It was however re-magnetized, and the directive 
force was now found to be increased to 1720. 
From these last experiments, it appears that the greatest 
directive force was given to the needle when the magnets 
were inclined to it in an angle not exceeding two or three 
degrees, and that this force is little, if at all, influenced by 
extent of surface ; as I conceive the small difference in favour 
of il e greater surface may be attributed to some difference 
in the quality of the steel, or its temper, both of which appear 
to have very considerable influence on the directive force. 
Two needles, the one five, and the other eight inches long, 
were cut out of the same plate of steel ; they were of equal 
weight, the short one being of greater width than the other. 
Being magnetized to saturation, their directive forces were as 
follow : 
