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. 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 “correspondence” between vane and dial is easily established. An advantage attaching to the use of a compass needle is that the magnet repels the wrong end of the needle.
[Illustration: FIG. 161.—General arrangement of electric wind recorder.]