Part 22 (1/2)
The post should be well painted, the top protected by a zinc disc laid under the top bracket, and the bottom, up to a point 6 inches above the ground level, protected by charring or by a coat of boiled tar, before the dial and the brackets for the vane rod to turn in are fastened on. A white dial and black arrow and letters will be most satisfactory against a dark background; and vice versa for a light background. The letters are of relatively little importance, as the position of the arrow will be sufficient indication.
It gives little trouble to affix to the top of the pole 4 arms, each carrying the initial of one of the cardinal points of the compa.s.s. The position of these relatively to the direction in which the dial will face must be carefully thought out before setting the position in the ground. In any case the help of a compa.s.s will be needed to decide which is the north.
Having set in the post and rammed the earth tightly round it, loosen the bracket supporting the vane rod so that the vane bevel clears the dial bevel. Turn the vane to true north, set the dial arrow also to north, and raise the bevel so that it meshes, and make the bracket tight.
Note.--In the vicinity of London true north is 15 degrees east of the magnetic north.
The pole must be long enough to raise the vane clear of any objects which might act as screens, and its length will therefore depend on its position.
As for the height of the dial above the ground, this must be left to individual preference or to circ.u.mstances. If conditions allow, it should be near enough to the ground to be examined easily with a lamp at night, as one of the chief advantages of the system is that the reading is independent of the visibility of the vane.
A Dial Indoors.--If some prominent part of the house, such as a chimney stack, be used to support the pole--which in such a case can be quite short--it is an easy matter to connect the vane with a dial indoors, provided that the rod can be run down an outside wall.
An Electrically Operated Dial.--Thanks to the electric current, it is possible to cause a wind vane, wherever it may be set, to work a dial situated anywhere indoors. A suggested method of effecting this is ill.u.s.trated in Figs. 158 to 161, which are sufficiently explicit to enable the reader to fill in details for himself.
[Ill.u.s.tration: FIG. 158.--Plan and elevation of electric contact on vane post.]
In-this case the vane is attached (Fig. 158) to a bra.s.s tube, closed at the upper end, and supported by a long spike stuck into the top of the pole. A little platform carries a bra.s.s 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 compa.s.s 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 bra.s.s pressing against the tube. This brush is connected with a wire running to one terminal of a battery near the dial.
[Ill.u.s.tration: 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 s.p.a.ced 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.
[Ill.u.s.tration: FIG. 160.--Another type of electric dial with compa.s.s needle for pointer.]
The exact.i.tude 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 compa.s.s 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 compa.s.s 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 compa.s.s needle is that the magnet repels the wrong end of the needle.
[Ill.u.s.tration: 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 s.p.a.ces 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.
For the expenditure of quite a small number of pence, one may construct a machine which will show fairly accurately what is the value of one's grip and the twisting, power of the arms; and, even if inaccurate, will serve for compet.i.tive purposes. The apparatus is very simple in principle, consisting of but five pieces of wood, an ordinary spring balance registering up to 40 lbs., and a couple of handles. The total cost is but a couple of s.h.i.+llings at the outside.
Fig. 162 is a plan of the machine as used for grip measuring. The base is a piece of deal 1 inch thick, 2 feet long, and 5-1/2 inches wide. The lever, L, is pivoted at P, attached to a spring balance at Q, and subjected to the pull of the hand at a point, R.