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on three levelling screws. The centres of each coil at the thin flange
end are hollowed out trumpet-shaped as much as possible, and tliese
two ends of the coils brought close together and fixed by ebonite
angle brackets on the outer ends of the coils. On an aluminium wire
are fixed three sets of little magnetic needles, together with the small
concave mirror at the top, as seen. The middle set can just move
freely in the hollowed coil centres, and forms an astatic combination
v.-ith the remaining two sets close to the top and bottom of the coils.
Fig. 140.
This magnet system is suspended by a fine silk fibre from a milled-
headed pin, which can move up or down in the top of a standard of
the shape shoivn, and a gdass shade protects these parts from air
draughts. It will now be seen that, when the coils are joined up so
that opposite polarities arc adjacent, the two outside sets of magnets
are affected by the outside convolutions of the two coils, and will
both be deflected in the same direction as the middle set. Thus
the deflecting couple will be that due to the sum of the couples acting



284



Apparatus.
on the ihrcc sets of needles individually. To obtain greater damping
due to air resistance, a light mica strip may be used instead of the
aluminium. To make the galvanometer ballistic use the wire, and in
addition weight the needle system so as to increase its periodic time
of oscillation.
Thomson High = resistance Astatic Galvanometer.
