Mr Christie on the Variation of the Magnetic Needle. 165 
l?rom the effects which I have observed to be produced on iron 
by its rotation, it appears probable, if the magnetism of these 
bodies be not caused by their rotation, that at least the effects 
will be modified by, and, to a certain extent, dependent on such 
rotation. Since first observing the fact, that simple rotation will 
cause a peculiar polarity, if I may be allowed the expression, in 
iron, I have made a great variety of experiments on the subject, 
which have enabled me to trace the laws according to which this 
polarity in the iron affects a magnetic needle, independently of 
the effect produced by the mass. It would lead me to too great 
a length here, to state the several effects that are produced by 
the rotation of iron, or the laws which govern them ; but I will 
briefly mention one. Let us imagine a plane to pass through 
the centre of an horizontal needle, at right angles to the meri- 
dian, and making an angle with the horizon equal to the dip ; 
then, if the plane of a circular plate of iron coincide with this 
plane, and the plate be fixed on an axis passing through its 
centre at right angles to its plane, so that it can be made to re- 
volvedn its own plane, the direction of the needle will be diffe- 
rent, according as the several points of the plate are brought in- 
to any particular position, by making it revolve in one direction 
or the opposite, excepting in four positions of the centre of the 
plate. If the centre of the plate be successively placed to the 
east or west of the centre of the needle in the same horizontal 
line, and over the needle in the plane of its meridian, then the 
deviation of the needle due to the rotation of the plate wall be in 
contrary directions in the two cases, the plate revolving in the 
same direction in both. These and other peculiar effects arise 
entirely from the rotation of the iron, and are not produced by 
any friction on the axis. As the effects are not very consider- 
able, to render them conspicuous, it is necessary to make use of 
a plate eighteen inches in diameter, and to have its centre within 
sixteen inches of that of the needle. If the needle is under the 
influence of magnets, as in the foregoing observations, the effects 
produced by the rotation of the plate are considerable *. 
* As the interesting statement in the above article has a reference to the 
important experiments of Mr Barlow, in Art. xi. of the present Number v we have 
used the liberty of inserting it. 
