are joined to the binding screws of the primary coil the contact-breaker will begin buzzing.

How to Make a Small Dynamo.

PART I.

The dynamo is not the most simple piece of mechanism extant, and I am inclined to think that many boys would find it rather a poser to make one. At the same time it is perfectly evident that there are heaps of our readers who are very anxious indeed to try, at all events, and as we must aim at more elaborate apparatus as we advance in electrical knowledge, it is a pity not to endeavor to supply them with the help they need.

Well, then, if, like Pears’ soap baby, they “won’t be happy till they get it,” I will do my level best to bring down the subject into the range of their capability. It will not cost them much to try the experiment, and if they don’t succeed they must not blame me, but their “vaulting ambition,” which has “o’erleapt itself.” There is no reason whatever why a boy who is accustomed to metal working should not succeed in making the small machine described if he first masters the principles of its construction.

The advantage of a dynamo, I may here remark, is that by its means we are able to produce a current of voltaic electricity at any moment by turning a wheel without bothering with acids or carbons, or zincs, or any other of the various articles necessitated by the use of a battery.

Furthermore, the current goes on as long as you turn the wheel, and stops directly you stop, there being no loss between whiles. Of course, both battery and dynamo have their advantages and disadvantages—nothing in this world being perfect all round—and for some purposes the dynamo is best, for others the battery. For example, it would be absurd to use a dynamo to ring an electric bell—not that it would not do it with tremendous energy, but in the case of a bell what one wants is merely to ring it for a few seconds at long intervals, and for this work a battery in which there is little current, but which is always ready to give that little without touching it, is facile princeps. But for experiments in which a strong continuous current is required, the dynamo comes to the front, as there is no “polarization” to detract from its value, as in the case of the battery. One does not always want to be messing with chemicals in setting up a battery, when one only requires the current for a short time, and the dynamo is always ready, and merely turning the handle produces the required current in a moment. Besides this, viewed merely in the light of a magneto-electric machine, it will give a considerable shock to any one who holds two handles fixed to its terminals.

Having now enumerated the advantages of the machine, it behooves me to endeavor to describe its various parts and the method of making them. There are several methods of dynamo-making, but that which seems to be the most used and most easily followed in the case of a small machine, is that of the type known as the “Siemens” dynamo, from the inventor of the armature, which is of peculiar construction.

The action of the dynamo depends on the fact that if a piece of soft iron is surrounded by a coil of insulated wire, when the soft iron is approached to a magnet it becomes itself a magnet, and at the same time a current is generated in the coil of insulated wire which surrounds it. This current is, however, of only momentary duration, and ceases if the soft iron remains stationary; but on removing the soft iron from the magnet another current is generated in the coil of wire, but this is a current of the opposite kind of electricity, and travels in the opposite direction to that produced in the former case. Now you have only to imagine that, by means of rotating in front of the poles of a magnet, a piece of soft iron is kept continually approaching and receding from the magnet, and that this soft iron is surrounded by wires in which circulate currents positive or negative according to the direction of the movement of the soft iron, and then, if we can arrange to carry off all the positive currents to one binding-screw, and all the negative currents to another binding-screw, we shall have a continuous current generated as long as the soft iron revolves. All this is practically carried out in the construction of the dynamo, and on the accuracy with which it is done the efficiency of the dynamo depends.

To make the base of the machine, take a piece of deal 5½ inches long by 3½ inches broad by ⅞ inch thick. This can be stained afterwards to make it look nicer; it must be planed well and polished up quite smooth.

Fig. 1.—Sectional Diagram of One Side of Magnet.

The dotted lines show position of coils of wire. A, One side of hole for armature.

The greatest difficulty of the whole business has now already to be confronted—viz., the manufacture of the magnet. This is almost invariably cast in two pieces, and for those who cannot make the castings there is no help for it but to have recourse to the ironmonger, or, better still, a practical electrician. The following instructions will then assist you to put the castings together:

Supposing this difficulty to have been overcome, and two pieces of soft iron to have been cast in the form of Fig. 1, both exactly the same size and shape; the next thing to do is to convert it into an electro-magnet by winding seven layers of No. 16 cotton covered wire over each leg, at the part shown by the dotted lines in the illustration.

The size of the legs of the magnet is as follows:—Total length from B to C, 4⅛ inches; thickness of top piece from B to D, ½ inch; length of top piece from B to D (half total length of top of magnet), ¾ inch; breadth of side of magnet all the way down, 1¾ inch; height from E to C, 1½ inch; thickness of the part between D and E, round which the wire is wound, ⅜ inch. When I say “breadth” in this description, I mean what you can’t see in the sectional drawing, because it recedes from you; when I say “thickness,” I mean what is shown in the drawing. It is necessary to explain this, as the terms are rather confusing. The ends of the sides between D and E are rounded to admit of the wire being more evenly wound on them.

Fig. 2.—Magnet Put Together.

A B C, Screws. D, Junction of two wires. E F, Ends of coils. H H H H, Holes for screws at end. The dotted lines show position of wire, and screws fastening magnet together and to base.

It is not essential to use a permanent magnet in this machine, as a certain amount of “residual” magnetism remains in the iron when once excited; and the coils of wire on the armature being acted on by the armature, which is slightly magnetized by this residual magnetism in the magnet, have a reactionary effect, and excite the armature, which excites the magnet afresh; and thus the magnet and its coils, and the armature and its coils, go on acting on