[Illustration: FIG. 158.--Plan and elevation of electric contact on vane
post.]
In-this case the vane is attached (Fig. 158) to a brass tube, closed at the
upper end, and supported by a long spike stuck into the top of the pole. A
little platform carries a brass ring, divided into as many insulated
segments as the points which the vane is to be able to register. Thus,
there will be eight segments if the half-points as well as the cardinal
points are to be shown on the dial. The centre of each of these segments
lies on a line running through the centre of the spike to the compass point
to which the segment belongs. The tube moves with it a rotating contact
piece, which rubs against the tops of the segments.
Below it is a "brush" of strip brass pressing against the tube. This brush
is connected with a wire running to one terminal of a battery near the
dial.
[Illustration: FIG. 159.--Magnetic recording dial.]
The Dial.--This may be either vertical or horizontal, provided that the
arrow is well balanced. The arrow, which should be of some light
non-magnetic material, such as cardboard or wood, carries on its lower
side, near the point, a piece of soft iron.
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