This section is from the book "The Boy Mechanic Vol. 2 1000 Things for Boys to Do", by Popular Mechanics Co.. Also available from Amazon: The Boy Mechanic, Vol2: 1000 Things for Boys to Do.
By Wm. H. DETTMAN
The construction of this instrument is so simple that any amateur can make one, and if accurate calibrations are desired, these can be marked by comparison with a standard anemometer, while both are placed in the wind.
The case of the indicator is built of thin wood - the material of an old cigar box will do - 9 in. long, in. wide and 1 1/2 in. deep. If cigar-box material is used, it must first be soaked in warm water to remove the paper. If a cover is to be used on the box, a slot, on an arc of a circle, must be cut through it to show the scale beneath. The arc is determined by the length of the needle from a center over the axis on which the needle swings. When the box is completed, smooth up the outside surface with fine sandpaper and give it a coat of stain.
The core of the magnet is made by winding several layers of bond paper around a pencil of sufficient size to make an inside diameter of slightly over 1/4 in., and a tube 2 in. long. Each layer of the paper is glued to the preceding layer.
Two flanges or disks are attached to the tube to form a spool for the wire. The disks are cut from thin wood, 1 1/4 in. square, and a hole bored through their centers so that each will fit on the tube tightly. One of them is glued to one end of the tube and the other fastened at a point 1/2 in- from the opposite end. The space between the disks is filled with seven layers of No. 22 gauge insulated magnet wire, allowing sufficient ends of the wire to project for connections. The finished coil is located in the box, as shown at A, Fig. 1.
The core for the coil is cut from a piece of 1/4-in, iron rod, 1 1/4 in. long, and a slot is cut in each end, 1/4 in. deep, into which brass strips are inserted and soldered, or otherwise fastened. The strips of brass are 3/16in.
wide, one 1 1/2 in. long and the other 3/4 in. Two 1/16-in. holes are drilled in the end of the long piece, and one 1/16-in hole in the end of the short piece. The complete core with the brass ends is shown in Fig. 2.
Ill: The Indicator Box with Coil, Needle and Scale, as It is Used in Connection with the Anemometer
The needle B, Fig. 1, is made of a copper or brass wire, about 6 in. long, and is mounted on an axis at C. The detail of the bearing for the axis is shown in Fig. 3. The axis D is a piece of wood fitted in the U-shaped piece of brass and made to turn on brads as bearings, the center being pierced to receive the end of the needle. After locating the bearing for the axis C, Fig. 1, it is fastened in place so that the upper end or pointer of the needle will travel over the scale. The needle is then attached to the bearing after having been passed through the inner hole of the longer brass strip of the core, and the coil is fitted with the core in the manner shown at D. A light brass coil spring is attached to each end of the core, as shown at E and F, the latter being held with a string, G,
Ill: Fig 2 Fig 3
The Metal Core for the Coil and the Bearing Block for the Axis of the Needle whose end is tied to a brad on the outside of the box, for adjustment. A better device could be substituted by attaching the end of the spring F to a nut and using a knurled-head bolt passed through the box side. One of the wires from the coil is attached to a push button, H, to be used when a reading of the instrument is made. The connections for the instrument consist of one binding post and a push button.
The anemometer resembles a miniature windmill and is mounted on top of a building or support where it is fully exposed to the air currents. It differs from the windmill in that the revolving wheel is replaced by a cupped disk, A, Fig. 4, fitted with a sliding metal shaft, B, which is supported on crosspieces, CC, between the main frame pieces DD. The latter pieces carry a vane at the opposite end. The frame pieces are 1/2 in. thick, 2 1/4 in.
wide and 36 in. long, and the cross-pieces have the same width and thickness and are 4 in. long.
Ill: The Anemometer as It is Mounted on a Standard Similar to a Small Windmill Weather Vane
A variable-resistance coil, E, is made as follows and fastened in the main frame. The core of this coil is a piece of wood, 2 in. square and 4 in. long, and wound with No. 18 gauge single-wound cotton-covered german-silver wire. The winding should begin 1/4in. from one end of the core and finish 1/4 in. from the other, making the length of the coil 3 1/2 in. The ends of the wire are secured by winding them around the heads of brads driven into the core. A small portion of the insulation is removed from the wire on one side of the coil. This may be done with a piece of emery cloth or sandpaper. A sliding spring contact, F, is attached to the sliding shaft B, the end of which is pressed firmly on the bared portion of the wire coil. One end of a coil spring, which is slipped on the shaft between the pieces CC, is attached to the end crosspiece, and the other end is fastened to the sliding shaft so as to keep the shaft and disk out, and the flange H against the second crosspiece, when there is no air current applied to the disk A. The insulation of the standard upon which the anemometer turns FIG.5 is shown in Fig. 5. The standard J is made of a piece of 1/2-in.pipe, suitably and rigidly attached to the building or support, and the upper end, around which the anemometer revolves to keep in the direction of the air currents, is fitted with a plug of wood to insulate the 1/4-in. brass rod K. A bearing and electric-wire connection plate, L, is made of brass, 1/8 in. thick, 2 in. wide and 4 in. long. The bearing and connection plate M are made in a similar manner. The surface of the holes in these plates, bearing against the pipe J and the brass rod K, make the two connections for the wires from the variable-resistance coil E, Fig. 4, located on the main frame, to the wire connections between the two instruments. These wires should be weather-proof, insulated, attached as shown, and running to and connecting the indicator with the anemometer at NN, Fig. 1.
Two or more dry cells must be connected in the line, and when a reading is desired. the button H, Fig. 1, is pushed, which causes the current to flow through the lines and draw the magnet core D in the coil, in proportion to the magnetic force induced by the amount of current passing through the resistance in the coils on E, Fig. 4, from the contact into which the spring F is brought by the wind pressure on the disk A.