266 
THE TELEGRAPH. 
which is carried down into the centre of the lower coil, where 
it carries a second small magnet placed astatically to the 
former. Perpendicular to this magnet, in the horizontal axis 
of the lower coil two discs of mica are fixed to the aluminium 
rod, so that they may render the movements steady by their 
resistance to the air. 
The slender aluminium stem to which the whole mobile 
system is attached, is suspended to the upper part of the 
apparatus by a very fine silk fibre, in the very centre of the 
apparatus. There are levelling screws in the stand, so that 
the magnets can be adjusted for free movement, the correct 
position being indicated by a spirit-level. 
The rays reflected from the mirror are brought to a focus 
either by a convex lens or by the concavity of the little mirror. 
These rays proceed from a lamp 30 inches distant, and passing 
through a vertical slit at the centre of the graduated scale, 
are reflected by the mirror and form an image of the slit upon 
the scale. This image moves from right to left according to 
the deflection of the magnet, and as the angle through which 
it moves is double that of the mirror, the effect is equal to a 
weightless index nearly 5 feet long, so that the slightest deflec* 
tions of the magnet are readily perceptible. And further, as a 
very small deflection ranges over the whole scale, the number 
