supporting spring.
D, Strip of brass supporting screw.
S, Screw to adjust position of soft iron, I.

Current travels from the binding-screw to C, from C to S, thence to D and round the coil by the wire, returning to battery by screw in opposite corner.

A narrow strip of stout brass is now taken and bent at right angles near one end, so that when screwed down to the base-board by holes in the smallest leg the longest leg will stand upright. Stand it up on the base in front of the coil and note a point on the strip exactly opposite the core. Make a hole through this point large enough to admit a small screw used on paper fasteners. Now take the flange part of the paper-fastener and solder it to the back of the brass strip, so that the screw will work through both (Fig. 5). This is done to avoid the trouble of making a flange in the strip, but if you can do this, so much the better.

Now, the coil having been fastened to the base by fine screws through it into the ends of the reel, nearly in the center of the base, we must find a place on the base in a straight line with the end of the core (as at C, Fig. 6), and here we fasten another piece of bent brass similar to the last. The end of the contact breaker is now soldered to this brass strip in such a way that the piece of soft iron at the other end is exactly opposite the core and about ¹⁄₁₆ inch distant from it. The screw of the paper fastener must now be tipped with platinum by cutting off the end and drilling a fine hole in it, in which hole a small piece of platinum wire can be soldered. The amount of wire and foil required, although very minute, will cost you about twenty-five cents, platinum being a very expensive substance. It can be bought from a chemist or electrician.

The screw having been prepared in this way, we must next fasten the brass strip to which the flange is soldered upright on the base, so that the platinum point of the screw, when inserted, will just come in contact with the square of foil on the spring. By turning the head of the screw the soft iron can thus be forced nearer the core, and the rapidity of its vibration is thus controlled. The coil is now complete, except the connections, which are made (preferably underneath the base by letting the wires through) by joining the ends of the thin wire to two “binding screws,” which are made for this purpose and can be obtained from the dealer. One end of the thick wire of the coil is fastened to the strip of brass supporting the contact-breaker, the other end is fastened to a binding-screw on one side of the base—the strip of brass supporting the screw being connected by a wire with another binding-screw on the other side. This sounds rather intricate, but will easily be understood if we consider that the current from the battery when the wires are connected with the binding-screw must pass through the brass strip to the screw, thence through the contact-breaker to the coil, and, having passed round the coil, back to the battery through the binding-screw attached to the other end of the wire. (See Fig. 6.)

It is now evident that when the contact-breaker is in contact with the screw a current will pass through the primary coil, and will cause the soft iron core to become a magnet and thus attract the soft iron. When this moves towards the magnet, contact is broken and the core is instantly demagnetized, so that the spring flies back and contact is made again. The screw is adjusted so that the contact is broken just as the soft iron touches the core. When the battery is joined on, the contact-breaker will fly backwards and forwards with tremendous speed, making a loud, buzzing noise, while brilliant sparks will appear between the platinum wire and foil.

In order to feel the effect of the shock, two handles will be required; these can be made by simply bending two pieces of tin about two inches by four inches round a ruler and neatly soldering the joins. A wire is now fastened to the end of each tube, the other ends being inserted in the binding screws connected with the thin wire of the secondary coil, which are at the opposite corners of the base to those which are joined to the ends of the primary coil. When the coil is buzzing, if these handles are tightly held, a powerful shock will be felt, in fact, a weak battery only should be used with the coil of the dimensions given, or it may be impossible to release the handles, and this is too strong to be pleasant.

The current can be regulated by means of a “regulating tube,” that is simply a brass tube which is made to slip over the core between it and the primary coil; the farther the tube is pushed over the core, the less powerful the shock. The dimensions of the coil being the same, a little ingenuity will enable any one to affix a regulating-tube. I will only say that instead of winding the coil direct on the core a tube of brown paper is formed a little larger than the core, and on this the wire is wound. Between this tube and the core the brass tube is arranged to slip in and out, the hole in the end of the reel farthest from the contact-breaker being made larger for its accommodation.

This concludes my description of the coil, but perhaps a few hints as to suitable batteries may be useful. If a strong battery which will only work the coil for a short time is required, the bottle bichromate is a good one. It can be bought from a dealer, or one can be made in a simple form by taking a jar and filling it with a strong solution of bichromate of potassium, to which a little sulphuric acid has been added. Take two pieces of gas carbon and three pieces of sheet zinc, both cut to the right size to dip in the solution to the bottom of the jar.

At the top of the zincs and carbons bore small holes, and below these place narrow strips of wood to keep them apart when in use; these must be long enough to reach across the top of the jar when the zincs and carbons are in the solution.

Arrange them thus: zinc, wood, carbon, wood, zinc, wood, carbon, wood, zinc; bind them lightly together by means of two more pieces of wood placed outside the outer zincs, and the whole tied together with string. Connect the three zincs together with one piece of wire, and the two carbons with another, taking care that the wire connecting the zincs, does not come in contact with the wire connecting the carbons. To one zinc attach a piece of covered wire, and to one carbon attach another, these two wires are connected with the binding screws of the primary coil. This battery is extremely strong, double as strong as the bottle bichromates sold, as there are more zincs and carbons employed, but it only lasts a short time before needing to be replenished.

Daniell’s battery is a weaker form, but lasts much longer, say for two or three hours in constant work. Take a deep jar and inside it place a porous jar of earthenware, which the electrician will provide. Now get a piece of sheet copper of the right size to go into the jar, and bend it round so that the porous jar will go inside it. A piece of sheet zinc will be wanted to go inside the porous jar. Both zinc and copper must be high enough to reach the level of the solutions when the jars are full. The porous jar is filled with dilute sulphuric acid, or solution of common salt; the jar outside is filled with “saturated” solution of sulphate of copper—that is, as strong as it can be made. Lumps of sulphate of copper are kept in the outer cell, which will keep the solution concentrated by slowly dissolving. Attach one wire to the zinc and another to the copper, and when these