will, immediately you connect the battery, pass from one to the other; and, bringing them nearer, the spark or flame will appear to be continuous. It is called the secondary spark, and is the result of the induced current.

3rd Experiment.-Attach the wires from the secondary terminals to the discharger, to the ends of which two fine wires ought to be attached. Connect the battery. By varying the distance between the ends of the discharge, you can see the extreme distance of air the spark will pass through: the length of spark will depend upon the amount of battery used. Vary the distance, and observe the different appearance of the spark, and its varying intensity. When the two ends are brought near each other, the spark becomes less blue; approach it still nearer, it becomes red, and is called a calorific spark, as, under that condition, it is more suitable for deflagrating experiments.

4th Experiment.-Arrange the discharger so that sparks of a good length pass between; bring the flame of a taper near: the spark will diverge towards the flame. Let the sparks pass through the flame: under this circumstance the distance may be increased. If the sparks pass through the upper part of the flame, they will appear as a white ball; if they pass through the lower part, a thin line will be the appearance.

5th Experiment.-Place some fine copper and steel filings on a card; pass the spark through them: the filings will be found separated.

6th Experiment. Arrange very fine copper filings alone on some paper: it will be found that one wire will attract the copper filings, whilst the other remains apparently inactive.

7th Experiment.-Substitute for the copper filings powdered plumbago on glass: a decided repulsion will ensue.

8th Experiment.--Place a plate of glass over some hot water so that it might be covered with vapour: on placing the wires from the secondary coil at some distance from each other, a long zigzag spark will pass from one wire to the other along the glass.

9th Experiment.-Place a piece of paper between the ends of the secondary wires: the paper will be punctured immediately the battery is connected. Instead of paper use gold-leaf: it will be immediately deflagrated.

10th Experiment.-Attach to the discharger two fine platinum wires or two iron wires: the wires will be ignited and deflagrated.

11th Experiment.-Try the deflagrating effects mentioned in page 652. 12th Experiment.-Place the two ends of the secondary wires one on each side of a piece of plate glass. Connect the battery: in a few minutes the glass will be fractured.

13th Experiment.-On the discharger table place a moistened piece of ebonite or gutta-percha: zigzag sparks, as in Experiment 8, will be seen. 14th Experiment.-Remove one wire from the discharger, and fix in its place a brass ball: the sparks will now assume a brush-like appearance. 15th Experiment.-Decompose water by inserting the two ends of the secondary coil in water: the gases may be collected by using proper apparatus. 16th Experiment.-Arrange the two wires of the discharger, and allow a stream of sparks to pass; bring in the stream a piece of gun-cotton, held by a pair of forceps: it will ignite.

17th Experiment.-Drop a little sulphuric ether upon a piece of cotton: it will ignite by bringing it in the stream.

18th Experiment.--Sprinkle gunpowder on cotton-wool, and bring it within the stream: the gunpowder will explode,

19th Experiment.-Upon the table place a glass plate with a small piece of phosphorus on it: on passing the spark through it, the phosphorus will ignite There are many other experiments that can be performed by ignition--firing of fuses, and so on.

20th Experiment.-Disconnect the secondary wires; attach the battery. On applying the knuckle to one of the supports, you will receive faint sparks and slight shocks. This is known as a static effect: no result from the other support. It will be found that these effects are only visible from the outer end of the secondary wire. Instead of trying with your knuckles, attach one end of a Geissler tube, it will be found partially illuminated; by placing the finger round the glass near the end, the light will be increased; pass the fingers slowly down, it will appear as if they attracted the light. This forms a very pretty experiment, and may be varied.

21st Experiment.-Attach the wire from the ball of a Leyden jar to the support giving static sparks; from the other support lead a wire to the outside of the jar; connect the battery, and the jar may be charged. So long as the battery contact is kept up the jar continues to receive charges; but it must be borne in mind that the secondary wire forms a complete circuit, and consequently acts as a discharger. According to this arrangement, a jar is constantly being charged and discharged.

It is possible to permanently charge a jar by connecting as before the outside wire of the secondary (that from which you get static effects); attach the other wire to the discharger, and again the discharger to the outside of the jar. Keep the points of the discharger some distance apart; connect the battery, and you will soon find the jar charged.

There are a variety of beautiful and pleasing experiments to be performed with the coil in passing the sparks through partial vacuum and through various gases.

22nd Experiment.-Repeat the experiments given on page 648, with the apparatus (Fig. 8).

Relative to the spark in the air, Professor Noad observes: "If the spark be attentively watched in the dark, it is seen to be surrounded with a sort of yellow-green atmosphere. It is generally of an ovoid form, and seems to be collected principally round the negative pole. When a steady current of air is thrown upon the spark taken between two metallic wires, the luminous atmosphere becomes expanded into a large mass of irregular violet-coloured flame, surrounded by brindles of rays, the spark itself not appearing to undergo any variation."

23rd Experiment.- Mr. Groves has rendered his name very celebrated for his experiments of the discharges in rarefied media, especially the peculiar effect of stratification in the globe experimented with before (Fig. 22). Exhaust it well and introduce some vapour of naphtha; connect the secondary wires: the discharge will now be stratified, and not in a stream as before.

A variety of experiments may be made with glass tubes containing highly rarefied gases and vapours. These were formerly made at Bonn, by M. Geissler, and are known everywhere as "Geissler tubes." They vary in form and give most beautiful and varied results. They can now be obtained at a low price at almost any philosophical instrument maker's.

There are numerous experiments that can be performed with an induction coil. I have contented myself with giving some of the principal amongst them; but if you wish to carry out more, you will find a number mentioned,

especially with Geissler tubes, in Dyer's "Intensity Coils," which I have already referred to.

In connecting a Geissler tube you will find no difficulty, as the platinum wire is continued outside. Connect the secondary wires, and then the battery. Put out the lamp or gas, and the tube will be seen in perfection.

Arrangements can be made for giving a number of people a series of shocks by attaching by wires to the secondary terminals metal handles, one of which must be grasped by one person, who holds another, and so on, until the last person grasps the other handle. Immediately the battery is connected, a series of shocks will be felt.

TO CONSTRUCT A GALVANOMETER.

In carrying out any experiments with current electricity, a galvanometer is a most valuable instrument, as it enables you to detect and observe any currents that may be flowing through a wire. The following represents a very simple instrument. (See Fig. 23.)

Construct a reel or bobbin of the shape shown in the diagram, and wind a quantity (according to the delicacy required) of fine No. 35 silk-covered copper wire round it; make the ends into a helix. In the centre of the bobbin should be fastened a small pivot for the magnetic needle to rest on. The needle should be well magnetized and a little shorter than the coil, so as to revolve freely; but, as it is necessary that the needle should lie in the same direction as the wire, it remains hid. In order, therefore, that its deviation should be known, a needle of copper or some light_material is fixed at right angles to it. The coil is fixed on a board, and the ends attached to the binding-screws. In front of the indicating needle is placed a scale of degrees (Fig. 23); the slight deviation of the magnetic needle inside will, therefore, be read off by the indicator. The delicacy of the galvanometer can be greatly increased by suspending the needle with a piece of unspun silk from a small curved brass rod, instead of allowing it to rest on a pivot. In this case it would be necessary to leave an opening in the centre of the bobbin, in order that the needle and silk might move freely from the point of suspension.

TO CONSTRUCT AN ELECTRO-MAGNET.

In page 660 I made allusion to electro-magnets. The following will be found very simple directions for making an electro-magnet of small size, but one that would support a weight of some pounds:

Get some stout iron wire or small rod about in. diameter, and 5 or 6 in. long, and bend it into a horse-shoe about 3 in. long, and 1 in. across (outside), keeping the same diameter all the way up. (Fig. 25.) Make two reels of wood,* with discs at each end of 1 in. diameter; let the centre of each be just large enough to admit the horse-shoe. Procure a small quantity of cotton-covered copper wire (about No. 22), and wind it on each reel; first pass the end through one disc and wind one reel full, then pass the other end through. Do the same with the second reel, but take particular care that you wind both wires the same way round, leaving the ends of the wires out at corresponding discs. When the reels are wound, pass the horse-shoc through them, and join together the two inner wires.

For this empty crochet-cotton reels will do by increasing the size of the hole.

On completing the battery circuit with the outside ends, you will find your horse-shoe turned into a powerful magnet. Construct a small armature with a hook, so that you may attach weights.

An ordinary electro-magnet can be made by simply winding some cottoncovered wire round the naked ends of the iron horse-shoe-about two layers round each; varnish the whole with sealing-wax varnish, and you will have a capital magnet. These, I need hardly say, you can make of any size.

TO CONSTRUCT AN ELECTRIC BELL.

An electric bell may be constructed, similar to the one in the diagram, very cheaply. (Fig. 30.)

Make an electro-magnet in the same manner as above described (Fig. 25); use a similar horse-shoe, but rather smaller reels, and upon them wind in the same manner until full a quantity of 24 cotton or silk-covered wire; upon a small piece of mahogany fix this coil by means of the clamp and screw, C;

lead the ends of the wire to A terminal, and to the spring, s. At the place, H, is attached with a spring the armature, bent at its lower end and provided with a round knob for striking the bell, which should be fixed as drawn. The terminal, B, should be connected with the brasswork to which the hammer is fixed. The spring, S, you will see is resting against the armature, and serves the purpose of a contact-breaker similar to that described for the induction coil.

As soon as battery connection is made, the bell begins to ring, and continues a rapid noise so long as the battery is on. This bell is called a "trembling bell." Connect the two points, S and H, and it will become a single-stroke bell, that is, it will strike once for each time you make battery contact.

By lengthening the two wires, you can make the communication all over a house. Place the bell and the battery down below, and carry up two wires to any place: one from the battery, the other from the bell-the other batterywire being connected to the bell. Now, if at any place you join these two wires together, the bell will ring.

Fig. 31 shows an upright single-stroke bell. The clectro-magnet is made the same as in Fig. 30, but the bell-dome is fixed on an upright in the centre of

the magnet; the hammer is kept up by a spring. The dotted lines show the position of the hammer when the armature is attracted.

FIG. 31.

For bringing the two wires (wires making connection) the following diagrams shows very simple forms:

The first is what is termed a "button; " the second a "key." In the button, b and c represent the two wires. On pressing A, connection is at once made.

In Fig. 33 the two wires are represented by the brass key, A, kept up by a spring against a small bridge, and the stud, B. Immediately A is depressed, the two wires are joined together, and the bell rings.