gnetic 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. Under the path of this piece is
a ring of equally spaced magnets, their number equaling that of, the
segments on the vane. Between arrow and magnets is the dial on which the
points are marked (Fig. 159).

Each segment is connected by a separate wire with the corresponding dial
magnet, and each of these, through a common wire and switch, with the other
terminal of the battery (Fig. 161).

In order to ascertain the quarter of the wind, the switch is closed. The
magnet which is energized will attract the needle to it, showing in what
direction the vane is pointing. To prevent misreading, the dial may be
covered by a flap the raising of which closes the battery circuit. A spring
should be arranged to close the flap when the hand is removed, to prevent
waste of current.

[Illustration: FIG. 160.--Another type of electric dial with compass
needle for pointer.]

The exactitude of the indication given by the arrow depends on the number
of vane segments used. If these are only four, a N. read- ing will be given
by any position of the vane between N.E. and N.W.; if eight, N. will mean
anything between N.N.E. and N.N.W. Telephone cables, containing any desired
number of insulated wires, each covered by a braiding of a distinctive
colour, can be obtained at a cost only slightly exceeding that of an equal
total amount of single insulated wire. The cable form is to be preferred,
on account of its greater convenience in fixing.

The amount of battery power required depends on the length of the circuit
and the delicacy of the dial. If an ordinary compass needle be used, as
indicated in Fig. 160, very little current is needed. In this case the
magnets, which can be made of a couple of dozen turns of fine insulated
wire round a 1/8-in soft iron bar, should be arranged spokewise round the
compass case, and care must be taken that all the cores are wound in the
same direction, so as to have the same polarity. Otherwise some will
attract the N. end of the needle and others repel it. The direction of the
current flow through the circuit will decide the polarity of the magnets,
so that, if one end of the needle be furnished with a little paper
arrow-head, the "corr