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.]

The brush and segments must be protected from he weather by a cover, either attached to the segment platform or to the tube on which the vane is mounted.

The spaces between the segments must be filled in flush with some non-conducting material, such as fibre, vulcanite, or sealing-wax; and be very slightly wider than the end of the contact arm, so that two segments may not be in circuit simultaneously.  In certain positions of the vane no contact will be made, but, as the vane is motionless only when there is no wind or none to speak of, this is a small matter.

XXIX.  A STRENGTH-TESTING MACHINE.

The penny-in-the-slot strength-testing machine is popular among men and boys, presumably because many of them like to show other people what their muscles are capable of, and the opportunity of proving it on a graduated dial is therefore tempting, especially if there be a possibility of recovering the penny by an unusually good performance.