The experiments that may be performed with an induction coil are to some extent dependent upon the spark capacity of the coil, but as many attractive and interesting ones require a coil giving only a 1/4 in. spark, the owners of small coils have no reason to be discouraged because of the lack of a large coil. For X-ray work a 4 in. spark coil is about the smallest with which radiographs of the bones of the hand can be satisfactorily obtained, and a coil of much greater capacity is generally used. A 4-in. induction coil in series with a Tesla coil gives beautiful results, but this branch of coil work must be held for a future series of articles.

The vacuum or " Geissler" tube is at once the most interesting and spectacular accessory of an induction coil, and owing to the great variety of such tubes, can profitably receive considerable attention from the amateur experimenter. These tubes are made of thin glass in a multitude of shapes and sizes, with platinum electrodes sealed into the ends and the enclosed space partially exhausted of air, or the air replaced by certain rarified gases, each one having its own peculiar color of incandesence when excited by the current from a coil. For instance, nitrogen will give a reddish light at the positive end, and violet or light blue a the negative end. With hydrogen the colors are crimson and blue, while carbonic acid gas gives green and lavender blue, but arranged in rings or discs.

Compound tubes are also made, in which the tube having the electrodes is enclosed in another tube, the latter generally straight, and the space between the two tubes is filled with a colored fluorescent solution, which gives a beautiful tint to the light within the inner tube. Another type of tubes has the form of a large bulb, and contains what is seemingly a group of dull clay figures or spray of flowers with leaves, but which assume most beautiful colors when connected to the coil. The latter are quite expensive, however, and are not commonly seen, but are well worth the having by those who can afford them.

The size of tubes varies from the small, single tubes 3 in. long, to the large, compound tubes 15 in. long. The small ones can be made luminous with a coil giving only a 1/8-in. spark, but a 1/4-in. spark is necessary to secure full brilliancy. An easy way of estimating the spark requirements is to allow from three to four feet of tube for each inch of spark, much depending on the " fatness " of the spark. The writer has a frame with six 12-in. tubes in series, which are very brilliant with the spark gap on the coil set at 1 inch.

If the electrodes of the tubes become quite hot with continued operation, the spark gap is excessive, or the connection have too much resistance. The connecting terminals of tubes are quite easily broken off, and to avoid such breakage it is advisable to use stranded wire, such as is used for electric drop lights. The strands are easily twisted around the loops of the tubes, and a better connection obtained than with magnet wire. In the event of the terminal of a tube gassiot's cascade.

being broken off, it can easily be repaired by placing a small bead of tin foil, softly rolled up, inside the cup shaped terminal, pressing same against the end of the platinum electrode and fastening with sealing wax, shellac or liquid glue applied around the edges.

Induction Coil Making For Amateurs VII Experiments 279

Still another form of tube is that known as "Gassiot's cascade," the form being shown in the accompanying illustration. It was originally performed with table glasses under a bell glass of an air pump, but the special parts can now be imported made up ready for use. It makes a most striking class-room demonstration if the windows are shaded to darken the room, and would also make a fine window display after sunset. It consists of a large glass bulb enclosing a glass goblet which rests upon projections on the shoulders of the bulb to give a little space between the base of the goblet and the walls of the bulb. The cover has a neck projecting down nearly to the bottom of the goblet, and a platinum wire electrode fused into the top. In the lower bulb is another electrode, and and is supported by a suitable base. A partial vacuum is obtained with an air pump, and the electrodes connected to the coil, when the light seems to flow in a stream from the. upper electrode down through the neck into the goblet, which appears to quickly fill and overflow around the top, down the sides into the lower bulb to the other electrode.

An interesting experiment which impresses those unacquainted with high tension effects, is to take hold of one end of a vacuum tube, place the other end of tube on the positive discharging rods of the coil, pointing with the other hand to the negative discharger, the discharge gap being about 1/4 in. As the moving finger draws near to the discharger, a slight shock will be felt in the arms and the tube will begin to glow, the brightness increasing the nearer the finger is brought to the negative discharger, and fainter as the hand is drawn back. The end of the free finger should not be brought into actual contact with the dischaiger, as a smart shock will be given should this occur.

Induction Coil Making For Amateurs VII Experiments 280

Decomposition Of Water

An incaudescent lamp will also glow under the same conditions, whether it has a filiament or not, by touching one terminal of the lamp to the positive discharge. "With a large coil it is not even necessary to have actual contact; placing the lamp between the dischargers will cause it to glow.

If the coil be fitted with dischargers having sharpened points, paper, cardboard, glass and other sub-stances may be perforated. Place the piece of card-board between the points and start the coil; sparks will pass through, forming round, slightly charred holes. The gap between the points should be about one-half that of the coil capacity. To pierce glass it • is necessary to support it so that it remains fixed and the spark continuously applied to the same point. If the spark plays around the surface of the glass, make two deep rings of sealing wax and affix to either side of the glass. Place the points of the discharges within the wax rings and start the coil. The rings will hold the spark to one spot and a hole will soon be perforated.

A miniature arc light can be made from a lead pencil having a large, soft lead. Cut the point into two pieces, remove the wood from each end, wind several turns of No. 20 magnet wire around one end of each piece and connect to the dischargers. Place the pieces of pencil on two tumblers and bring the free ends nearly together; start the coil and then adjust the gap until the arc is formed, when a brilliant white light will be given off.

Using the pointed dischargers, twist around the ends pieces of fine iron wire, bring the free ends of the wire together till sparks pass freely, and until one wire becomes white hot, when it will burn brightly, emitting sparks of burning metal. Substitute copper wire for the iron wire and note the change in the color of the light.

Prepare metal filings with a clean, coarse file sprinkle a layer of fine filings upon a sheet of ebonite, start the coil and bring the discharger points to opposite edges of the filings. Part of the filings will be fired and the spark will have a zigzag shape with color varying with the metal. Copper, nickel and silver coins can be used to furnish filings of these metals. Strips of metal foil of the various kinds used for signs will be consumed in a brilliant flash, the color varying with the metal.

Take a discarded photographic plate and upon it attach with shellac a piece of tin foil cut to the shape of of a star or diamond. When the shellac is dry cut lines through the foil with a knife. Place the plate so that the discharger points are near opposite points of the foil figure and start the coil, when sparks will appear at the knife marks. With large coils, large foil signs can be illuminated with striking effect, a flaming discharge being the most effective.

The superior conductivity of heated air is shown by bringing the flame of a lighted candle near the discharger points. Blow out the candle and hold the wick between the points when the candle will be relighted, provided the wick still has a red glow.

The decomposition of water is an old but interesting experiment, and the materials necessary are easily obtained. Two pieces of platinum wire about 1 in. long are soldered to two pieces of No. 18, waterproof insulated wire, the joint being wrapped with electricians' tape and then thickly shellacked, so that only the platinum wire is exposed. Bend the platinums' ends backward to form houks, and twist the outer ends of the insulated wire into spirals, so that when placed over the opposite sides of a glass tumbler the hooks will be a short distance above the bottom of the glass.

Fill the tumbler nearly full of water, strongly acidulated with sulphuric acid or vinegar. Obtain a disk of cork large enough to fill the top of the tumbler, bore two holes with centers about 1 in. apart to receive two 3 in. pieces of glass tubing 1/4 or3/8f in. diameter. Place the cork and tubes in the tumbler and arrange the hooks so the points will be inside the lower ends of the tubes. Connect the free ends of the copper wires to the coil dischargers and start the coil. Bubbles will rise in the tubes, oxygen gas rising at the positive and hydrogen gas at the negative wire; the quantity of hydrogen gas will be about double that of the oxygen gas.

As frequent mention has been made of positive and negative discharger, it will be desirable to have a convenient way of determining which these are. Obtain some white photographic blotting paper, as this kind is free from interfering chemicals, and soak in thin, hot washing starch. After the starch is dry, dip in a solution of iodide of potassium, one ounce, and water one pint, and dry. To use, dip in water, place upon a sheet of clear glass and place between the dischargers. Start the coil and the paper around the positive pole will turn to a jurple or brown shade.