I 
l o 6 Captain Kater on the best kind of steel 
ject of experiment. The needle being in the magnetic meri- 
dian when the wire has no torsion, is afterwards forced to 
deviate from it to a mark distant about 6o°, by turning the 
index, and consequently twisting the wire. The number of 
degrees passed over by the index will be as the directive force 
of the needle. 
The needles which I have described were first made soft, 
and then hardened merely at their ends ; they were not po- 
lished, and were magnetized to saturation. 
Experiment l. 
Needles soft, and then hardened at the 
ends 
Weight of 
needle. 
Directive 
force. 
Blister steel, solid ellipse 
66 
5°o 
— ,open ellipse 
66 
520 
Spur steel, solid ellipse 
66 
54° 
, open ellipse 
66 
5 zo 
Shear steel, rhombus 
4 5 
435 
— — — — , rhombus , with 1 
cross piece of brass J 
45 
435 
By the experiments on magnetism made by M. Coulomb, 
it appears, that the directive forces of needles of similar form 
are to each other as. their masses ; the directive force, therefore, 
of a needle of the form of a pierced rhombus of 66 grains, 
would be expressed, according to the preceding experiments, 
by 638 . 
From many other experiments, which I regret were not 
registered at the time, it appeared that shear steel was capa- 
ble of receiving the greater magnetic force, and that the 
