I 
j 
114 Captain Kater on the best kind, of steel 
Experiment 12. 
Needles re-magnetized, 
Directive 
force. 
Parallelogram 
Rhombus 
Pierced rhombus 
1140 
955 
1069 
I now to my surprise found that the directive force, instead 
of increasing, had lessened in each of the needles, and I be- 
came anxious to discover the cause of so unexpected a phe- 
nomenon. It has been observed by M. Coulomb, and more 
fully entered into by Biot, that if a needle be magnetized to 
saturation by strong magnets, and afterwards weaker mag- 
nets be applied, the needle will lose some part of the force it 
had before acquired. Now, if in using the same set of mag- 
/ 
nets a certain degree of force be communicated to a needle, 
and the magnets be afterwards arranged in a manner less 
favourable for imparting magnetic force, it should seem, that 
this second operation would produce the same effect as would 
follow the use of magnets of less force, and that the magnet- 
ism of the needle would suffer a diminution. 
The method I had employed in magnetizing the needles, 
was that of Du Hamel, by joining the opposite poles of the 
magnets, and placing them on the centre of the needle, so 
inclined that each formed an angle, as I afterwards ascertained, 
of about 30 degrees with the horizon. The magnets were 
then slid from the centre to the extremities of the needle, and 
their poles being again joined at a distance from the needle, 
the operation was repeated. 
As I could in no way account for the diminution of directive 
