Oscar N. Dame.

As many readers of this magazine are the possessors of spark coils giving from one to two inch sparks, they will be interested in learning how, during the winter holiday season, they can utilize their coils to profitable advantage by arranging and letting to enterprising merchants the window display here described, which is exceptionally attractive at all hours except bright daylight.

In addition to the coil batteries, etc., it will be necessary to have three or four vacuum tubes, "Geissler" tubes, as they are frequently called, a few shapes being shown in Fig. 1. The smaller sizes are sold for about 50 cents each, increasing in price to $1.50 for 12 in. compound tubes; the latter giving the most beauti-ful floresence when excited by a coil giving a 1 in. spark. These tubes are exhausted of air and then partially filled with gases which give characteristic colors when excited by the coil. With carbonic acid gas the color is whitish green; hydrogen, white and red; nitrogen, orange yellow. The compound tubes are composed of an inner tube with the usual twists and ornamental turns, and an outer, straight tube which serves to protect the inner one against break age. This outer tube is also filled with various colored liquids which increase the luminous effect. As these tubes are almost entirely of foreign manufacture and imported by but few electrical supply houses, they will have to be ordered by mail by those not living in the largest cities.

The tubes are fitted at the ends with wire loops firmly set in the glass. They must be handled carefully, however, both to avoid breaking the glass and the outer ends of the loops. Simply mounted upon a frame with a black background, the appearance of the tubes is very fine, yet to get the best effects they should be rotated and a suitable device for doing this will new be described. Such an exhibit placed in a store window will attract much attention from passers, giving an excellent advertisement to the goods displayed in the window.

Referring to Fig. 2, the baseboard B is 12 in. square and £ in. thick. Holes for the supports, s, are cut 1 1/2 in. from the front edge. These holes are 1 1/2 x £ in. and are cut vertical, although the supports are inclined inward to be only 3 in. apart at the top. The supports, s, are 22 in. long, 2 1/2 in. wide and £ in long, those at the bottom | in. long. They should be carefully fitted and strongly secured by wedging and glue, as the revolutions of the wheel are liable to cause the frame to sway if poorly joined, the effects of which might be disastrous.

Fig. 1.

The piece C is 6 in. long, 2\ in. wide and 2 in. thick. Mortises for the supports are made 3 in. apart, and a hole bored through the center for the shaft I. This shaft is preferably made from a piece of 3/8 in. cold rolled steel, bushed with brass tubing, in which case the hole should be about 7-16 to 1/2 in., depending on the thickness of the tubing. Obtain the tubing before boring the hole, as it should be a drive fit. Care should also be used to see that the axis of the hole is exactly horizontal.

It will be necessary to have the outer end of the shaft threaded to receive nuts, one on each side of the revolving frame, which are screwed up hard and prevent the frame from turning on the shaft. Two collars are also needed. These can be made from the pieces of steel tubing, or 3/4 in. rod, with a 3/8 in. hole drilled in the center and set screws fitted to it. These are put, one on each side of the block, C. A pulley is mounted on the rear end of the shaft. A suitable pulley can be made of a pulley wheel used in awning pulleys and sold by large hardware dealers.

The revolving frame is made from two pieces, F, 36 X2 X 3/4, crossed with a halved joint. A hole for the shaft is bored at the center. At the back on each arm and 5 in. from the shaft glue blocks 2 x 3/4 x | in., and to these glue a wooden disk, D, 12 in. in diameter and about | in. thick. The bottom of a peach basket will serve, or it can be cut out with a fret or keyhole saw from a board. In the center a hole is bored for the shaft, and around the rim a band of strip brass is attached with shellac, drilling a few holes for a few short brass brads with flat heads which should be filed flat so as not to project and wear out the brushes. This brass tire should be made up and the ends soldered before putting on.

The front of the frame is then covered with thick cardboard, the front surface being black. If black cardboard is not easily obtained, use any color and cover with black paper or velveteen.

The tube holders shown in Fig. 4 are made up after purchasing the tubes, so that the fit will be accurate. The piece, II, is of a length to bring the two ends pieces at the narrow necks between the end tubes. The end pieces are nailed to the bottom strip, and have the outer ends cut out to fit the tube and covered with black velvet glued on. The tubes are held in place with fine wire, the ends of which are put around small wire nails put into either side of the ends. The tube frame is attached to the revolving frame by a wood screw at the center. This permits of turning the tube holders on the frame to obtain different effects when rotating.

The tubes are wired up as follows: On the top of the block, c, a brush is made of a strip of spring brass, 4 in. long and | in, wide. A quarter turn is made with pliers at the center and then bent at right angles. Holes are drilled or punched for two brass screws, by which one end is attached to the block so that the other end will bear firmly and securely on the shaft. A flexible wire is run from one of the screws to one end of the coil secondary.

Another brush is made from a strip 9 in. long with one end bent as previously mentioned, and the other end attached to one of the supports, S, as shown in

Fig. 2. See that the turned end bears on the rim of the disk, D. This end is curved so that the frame may revolve in either direction. This bearing is connected to the other secondary terminal of the coil. The tubes are then connected with magnet wire, as per Fig. 3, the wire being run on the back of the frame and brought through to the front at the ends of the tubes. Use care to run the wire between the arms and the card-board, so that the space between the wire and the rim of the disk is as great as possible. The high potential of the current will cause it to take the easiest circuit, and it is quite necessary that the distance between wires be at least double the sparking capacity of the current.

The frame is revolved by a suitable motor attached to the baseboard as shown in Fig. 2, and connected with a 1/4 in. round belt to pulley on the shaft. Several cells of battery will be needed for current, and a switch should be placed on the baseboard to facilitate operations. The frame should be started by hand when switching on the motor, as the weight is considerable and much more power is required to start it than to keep it going.

The coil required to operate this device with four 12-in. tubes need be only about 1/2-in. spark capacity, as but little current is required to bring these tubes to full brilliancy. Coils of larger capacity can be used, the battery being cut down to reduce the sparking length. A little experimenting will undoubtedly be necessary to determine just the right battery power to use, being careful not to use too much and burn out the wire terminals of the tubes.

Care must also be used in wiring up and connecting to the metal tips of the tubes, as the construction is not always of the best and the tips easily break off, making connections difficult. If unused to blowpipe work and soft solder, breaks of this kind had best be repaired by a jeweller.

In addition to being an excellent window dislay, this devices makes a very interesting display for the home, and as the cost is not heavy for the whole outfit, providing the coil be home-made, those interested in coil work will find it worth the making.