Frank W. Powers
With all except the very smallest of coils, it will frequently be found desirable to have a current revers-er by means of which the direction of the current through the primary winding may be quickly changed. In using Geisler tubes it is quite an interesting experiment to thus change the current, and note the color changes at the terminals of the tubes.
There are two forms of reversers in common use; the three point, two arm switch, and the ellipsoidal form having two brass contact surfaces, and the circuits completed through the shaft, shaft bearings and flexible contact springs, as shown in the illustration. The dimensions here given are suitable for a coil giving a two-inch spark, and may be reduced for smaller coils, but need not be enlarged for coils giving up to eight-inch sparks. To turn out a finely finished job will require a lathe having a slide rest and chucks, but passable work may be done with a hand drill and taps.
The first work will require a piece of round hard rubber rod 7/8 in. long and 1 in. diameter. Find the exact center of each end and drill 3/8 in. holes 3/8 in. deep. In these holes is mounted the shaft, which is in two pieces, the intercepting section of the rubber serving to insulate the two sections. The shapes of the two sections of the shaft are given in the illustration and they are turned or filed out of a piece of round steel.
A piece of brass tubing, 1 in. inside diameter and 7/8 in. long is then slipped over the rubber; holes are drilled, countersunk and tapped for two 1/8 in. machine screws on opposite sides of each end. The screws must be long enough to reach into the shaft a short distance; the latter being drilled for same, after locating the points by boring the holes above mentioned. These screws serve the desirable purpose of keeping in place the sections of tubing and shaft ends.
When this work is completed, the heads of the screws are filed to conform to the surface of the tubing, which point should be kept in mind in counter-sinking the holes for same, so that the heads will not sink too "deep. The important point is to be able to file the heads of the screws down so that but little of the slot remains, just enough to take a screw driver, should it ever be necessary to take it apart. ' The larger end of the shaft is bent to a right angle and fitted with a hard rubber or composition handle. A suitable handle can be obtained from a cheap button hook. It is fastened to the shaft with shellac, after first roughing the shaft fitting some with a file.
The shaft should be so fitted to the rubber tubing that the handle will be exactly vertical when the springs rest easily upon the rubber; when turned horizontal in either direction the springs rest upon the brass faces.
The spring contacts are bent up from strips of spring brass, the top ends being bent as shown, and the bottom ends thrust out at right angles and holes drilled to receive the screws for binding posts.
In connecting up, one end of the primary is connected with the wire from the condenser to the vibrator. The positive and negative wires from the battery are connected to the binding posts. It will be evident that when the handle is thrown horizontally to one side the current will pass in one direction. When the handle is vertical no current will pass, and it will be found very handy when adjusting the apparatus, thus to be able to throw off the current without disturbing the connections, as well as effecting a considerable wear on the batteries.
In many experiments with a coil it is desirable to make use of discharging balls, and this is especially true of wireless telegraphy. To make such balls, obtain two brass curtain pole ends, which are made of spun brass and come fitted with screws for attaching to curtain poles. These screws must be removed, which may be done by sawing them off with a hack-saw, leaving the heads inside the balls, which is not objectionable, or the small caps surrounding the screws may be unsoldered with a blow pipe and the screws removed. The latter method should not be used, however, unless the worker is rather skilled in soldering, or the resoldering of the cap is done by a jeweller, which service may be obtained at small expense.
If the screws are sawed off the brass rods carrying the balls are inserted a short distance into the screw boles and fastened with solder. If the screws are removed by taking off the cap, the rods are first soldered to the caps and these in turn soldered in place on the balls. Whichever way is used, all excess solder should be removed with a small, smooth file, and the joint polished with fine emery cloth, as all abrasions and points serve to dissipate electricity. The appearance of the balls is improved by a coating of thin lacquer.
Rubber or composition handles are fitted to the outer ends of the rods, after passing the same through the secondary terminal posts. These handles may be taken from the large button hooks, as previously mentioned for the current reverser.
A word of caution may be desirable to novices in the use of coils: Do not undertake to adjust the spark gap or connected apparatus when the current) is on. The brush discharge from any, except the very small coils, is quite enough to give a severe shock. It is quite probable that in a moment of carelessness, however, most of us learn how true this is.