PART III.

While the current is passing you can try the following experiment, to prove that the wire is wound on all right. If it is not wound as described there will be two north poles or two south poles, instead of one north and one south. Suppose we decide to make the leg on which the wire comes from the outside of the magnet the north pole, the wire from this must be joined to the wire coming from the zinc end of the battery, and the other coming from the inside, between the poles, joined to the wire from the carbon end. Now if, while the current is passing, a magnetized needle is approached to each pole consecutively, and one end of it is attracted and the other repelled in each case, the wire is all right; if both are attracted something is wrong. The needle must have been really magnetized beforehand, or it will deceive you; you can easily test if it is so with an ordinary permanent magnet.

Having magnetized the soft iron in the way described, we now join up the wires to the binding screws, under the base, and, the pulley being fixed on to the axle of the armature opposite to the commutator, the machine is now ready for use. To rotate the armature at a high speed it is necessary to connect the pulley by an endless band with a large, heavy wheel which can be rotated by hand.

For continuous work, as we cannot always be turning the wheel, a small steam-engine or water-motor must be employed. Worked in this way, the machine I have described can be made to light 2 5 candle-power lamps of 6 volts, and give about 12 volts of current. This is not much, of course, but by enlarging the proportions of the various parts, you can make as large a dynamo as you like; only the power required to work it naturally increases considerably. This machine will do a great deal of the work of a battery--for example it will run an induction coil or an electro motor at full power. By connecting two brass handles to the binding-screws by wires, you will get a powerful shock if you hold them while some one turns the wheel connected with the pulley; in fact, the shock is too powerful, and the person turning the wheel must be prepared to stop when the victim has had enough. If these handles are dipped into a glass of water slightly acidulated with sulphuric acid (to enable the current to pass more freely), and the dynamo briskly turned, you will soon see bubbles rising from the handles--which must, of course, be placed separate from each other--consisting of oxygen and hydrogen gas, into which the water is being decomposed by the force of the current. Water being composed of two quantities of hydrogen gas to every one of oxygen, it follows that double as much hydrogen will come off the handle which evolves it as will come off the other of oxygen, and this you will soon see to be the case; the bubbles on the former being much more numerous than those on the latter.

Now take a 5 candle-power 6-volt electric lamp, and fasten it on to the wires coming from the binding-screws (removing the handles) by the platinum loops at the top. If the dynamo is now briskly turned, you will find that the lamp will light up well, and as long as the wheel is turned and the dynamo is buzzing, so long will the lamp continue to glow. By turning the dynamo by steam or water-motor we have, therefore, a means of producing a continuous light, which will not drop at the end of a few minutes as in the case of a battery. This is the method by which all public buildings, etc., are lighted.

There is said to be always sufficient residual magnetism in the soft iron core (at any rate if constructed of ordinary soft iron, not specially annealed) to act on the armature when revolved, and this, acting on the magnet, increases its magnetism so that they react on each other until the maximum effect of the dynamo is reached. This is the case with the majority of dynamos used for lighting, etc.; but if you are of an experimental turn of mind, and are possessed of a battery as well as the dynamo, you can try the effect of magnetizing the soft iron cores by sending a current from the battery through the coil.

To do this, disconnect the wires from the magnet-coil from the binding-screws, and connect them with the terminals of the battery. The whole current from the dynamo now comes from the armature, and you will find that this current is considerably increased, sparks flying about in all directions when the handles from the binding-screws are approached to each other or rubbed together. The water will now be decomposed much faster, and you will be able to light an additional lamp or two, according to the strength of the battery.

Fig. 11 gives an idea of the positions of the parts of the dynamo when complete; it is not an easy thing to draw, and I can only hope the rough sketch will be intelligible to my readers. The spring A is below the roller of contact breaker, and the spring B above it, the diagonal line on the roller representing the vacancy between the brass pieces covering the wood. The wires from the ends of the magnet-coil go through the base, round the bottoms of the pillars A and B, and join the other wire between the pillars and the binding-screws. The wire from the pole on which the wire comes from _outside_ the magnet is joined to the binding-screw A in the figure. The other wire comes from between the poles, and is joined to the other binding-screw. If you can find out, by means of a galvanometer, which binding-screw is conveying the _positive_ current, the wire from the _south_ pole of the magnet is to be joined to the wire from this, and that from the _north_ pole of the magnet to the wire conveying the _negative_ electricity.

Whenever you join the wires, be sure to scrape off all the insulating material, and twist them firmly together; a little solder is an improvement. Whenever the wires cross the iron work be sure the insulating material is quite sound at that point. It is a good plan to roll paraffined silk round the wires at these places. Cut grooves under the base, in which the wires may lie, or the dynamo will not stand evenly. The dark line in the middle of the top of magnet in Fig. 11 shows where the two parts join. They should be screwed up tightly together.

As a concluding illustration, I give a diagram of my own method of turning my dynamo (Fig. 12). On the leg of an ordinary table T is fixed the heavy iron wheel W, which has a groove cut in its circumference for the reception of an endless band B. These wheels may be obtained for a few shillings from any ironmonger, as they are made for various machines, such as laths, fret-saws, sewing-machines, etc. The wheel is held by an ordinary screw fixed into the leg of the table, and revolves on the screw. The endless band (tape will do) passes over the groove and over the pulley of the dynamo placed on the table above the wheel.

It is better to let the pulley of the dynamo project beyond the end of the base, as shown in Fig. 11, in order to be able to connect it with a wheel placed below it, if required.

The best results are produced from the dynamo when the resistance of the interpolar (_i. e._ the lamp, or whatever it may have to work) is equal to the internal resistance of the machine. It is sometimes required to send a current through a greater resistance than this, and then it becomes necessary to employ what is familiarly termed a "shunt." If one lamp of high resistance is coupled to the dynamo, the resistance may be too great for the current to get round the magnet in sufficient quantity to give the required electromotive force. Supposing that this is the case, we make a second pathway for it by joining on a piece of iron wire (about ten inches of No. 30) between the two binding-screws, the lamp being connected with the same binding-screws, only further off. The result of this is that the current goes round by the second pathway and excites the magnet more powerfully, and this, in its turn, excites the armature more strongly, and so on, until enough current is produced to light up the lamp. The resistance of the shunt required depends on the resistance of the lamp. If this is low no shunt will be required, if very high the resistance of the shunt must be lowered, or else enough current will not pass to magnetize the soft iron cores, and the dynamo will give no current. The lower the resistance of the shunt required, the less wire we use.

Some Toys Worked by Electricity.