R. A. R. Bennett

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 ? 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, 4? 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, ? 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 each other, and mutually building up each other’s current, until the maximum effect which the machine is capable of giving is produced.

Before winding on the wire, the legs of the magnet between D and E should be covered with a band of silk soaked in melted paraffin wax to increase the insulation. New and soft wire, of the highest conductivity, should be used. Old, rinky, and hard wire will not do.

Fig. 3.—Armature of Dynamo.

S S S S, Grooved cylinder of soft iron. K L, Axle of cylinder. The wire is wound across from end to end on one side, and then from end to end in the same direction on the other side. W W, Ends of wire. A B C D, Grooves in cylinder for thread to hold wires in position. F, Wooden cylinder fixed on axle.

The wire is wound upon the legs of the magnet in such a way that when put together as shown in Fig. 2 the coils are in opposite directions, so that if the magnet were straightened out, or the two portions placed end to end, one coil would be a prolongation of the other. This can be most easily done, in the case of this particular magnet, by winding each leg separately, and the end of the outer coil of wire of one can be joined to the end of the inner coil of wire of the other at D in the cut, the other ends of the coils being left loose as at E and F, these being long enough to go down under the base—say, about 3 inches long to allow for joining up.

The electro-magnet having been wound, may now be placed upright on the base, its two limbs fastened together by a screw at A. The magnet is now to be fastened to the base in the middle of its breadth, and about an inch from one end, by means of two screws at B and C, passing through the base into the legs of the magnet. Before it is fastened on, however, you had better drill two screw holes on each leg at H H H H in the figure, and four corresponding to them on the other side. We shall want eight screws to fit these holes presently.

Fig. 4.—Section of End Armature.

The circles show the position of the coils of wire in the grooves A and B. C D, Ends of soft iron cylinder.

The magnet having been fixed, we now have to construct the armature, which is the next most important part of the machine.

This consists of a soft iron cylinder with an axle passing through its center, as at K L in the illustration (Fig. 3), S S S S being the soft iron cylinder. This cylinder has a deep groove cut from end to end, or is cast in that shape, and round this groove the wire is wound. The wire is number 18, cotton or silk-covered. Begin at the point marked H in the diagram, and wind over and over, from end to end, until that side is full; then cross over to the other side, going from H to R, and wind that side also in the same direction. The ends of the wire are shown at W W, and they must be left about an inch or two inches long, as we shall want to connect them with the commutator presently.

The dimensions of the armature are as follows: Length of axle, 5½ inches; circumference of cylinder, 1 inch; length of cylinder, 2 inches; width of groove, ¾ inch. The axle is composed of a piece of steel rod rather more than ? inch in diameter. The axle must be very truly centered in the armature, and the armature must be accurately mounted, as it has to revolve at a high rate of speed in a very limited space, between the poles of the magnet.

As it is rather difficult to explain the construction of the armature, I give another illustration (Fig. 4) of a section of the armature, which will show how the wire is wound on the groove, and the shape of the grooves themselves.

At one end of the axle is fixed the driving-pulley P, while at the other has to be fixed a small wooden roller F, over which two pieces of sheet brass have been fastened, each reaching nearly half round the surface of the roller, so that two gaps are left between them. This forms part of the commutator; but before we come to that we must consider how the armature is to be fixed between the poles of the magnet.

PART II.

Fig. 5.—Support for Pulley End of Axle.

The dotted lines show position of holes for screws and axle. P P, Holes for screws.

Returning to Fig. 1, we must see that the groove A, which forms half the channel in which the armature is to revolve, is ? inch semi-circle. When the two sides are fixed together as in Fig. 2, the hole between the poles should be about an inch in circumference, and the wire must be wound on the armature so that it easily slips into the cavity G, which must be made quite smooth for it to revolve in. It will be seen from the dimensions given that in diameter the armature is only a little less than the cylindrical space between the poles of the magnet, and in length it is about the same as the width of the magnet. It would be an unfortunate occurrence if the wire was to slip off the armature while revolving at a high speed, and therefore it is necessary to keep it firmly in its place. This is done by filing four small notches in the soft iron of the armature at the points marked A B C D in Fig. 3. Some strong wire or small string is now wound lightly round the armature to hold the coils of wire in their proper place, the notches holding this wire or string from slipping off at the ends of the cylinder.

The armature is now to be fixed in its proper place between the poles of the magnet.

Fig. 6.—Support for Commutator End of Axle.

The dotted lines show position of holes for screws and axle. P P, Holes for screws.

To do this we shall want two supports for the axle. These are made of brass, shaped as in Figs. 5 and 6, 5 being the one at the pulley end of the axle, and 6 that at the other end. They are fastened by screws through the holes P P, into the holes H H H H in the bottom part of the side of the magnet, as previously shown in Fig. 2.

When the armature is fixed in its proper place it will appear as Fig. 7, this being a sectional diagram from above, and the top pieces of the magnet being omitted for simplicity’s sake.

Fig. 7.—Ground Plan of Magnet and Armature when put together.

M M, Magnet. P, Driving pulley. A, Armature. R, Roller of wood covered with brass. Top of magnet and springs of commutator omitted.

The brass of which the supports are made should be about ? inch thick, and must, of course, be drilled in the center with a hole to admit the axle of the armature. To keep it exactly in the right place while revolving, a piece of circular brass tube, with a bore the size of the hole made to admit the armature, should be soldered to the brass supports in front of the hole; that for the pulley end of the axle should be ½ inch long. One at the other end is not necessary, but looks neater; this may be about ¼ inch long—i. e. as long as the end of the axle projecting beyond the brass support.

This much having been accomplished, we have now to consider the “commutator,” which is a piece of apparatus by which all the currents proceeding from magnet and armature are sent in one direction, and thus, instead of counteracting each other, are made available for experiments.

To make this necessary adjunct to the dynamo, take a circular bar of brass rod about ? inch in diameter and an inch long. Into the middle of this solder a brass screw by drilling a hole and inserting its upper end minus the head. On this screw works a brass nut about ? inch long. At the other end of the rod a hole is drilled for the insertion of another brass screw, long enough to go through the base. Another pillar precisely like this has now to be made, only ½ inch high without the nut. Now cut two pieces of sheet brass 2 inches long and ½ inch broad, sufficiently stout to act as springs and not too stout to be elastic. At one end of each cut a longitudinal hole about ¾ inch long and ? inch broad; that is to say, this slit must be broad enough to slip over the top of the screws above the pillars. At the other ends of the brass springs slits of equal length, but very narrow—only about ¹/₂₄ inch wide—may be cut, to make the brass more “springy.” On the under side of this end of one spring and the upper side of the other, two pieces of thin sheet copper are fixed, the same breadth as the springs, and about ½ inch long. These are soldered by one end to the side of the spring, so as to act as springs themselves, their other ends being free.

All this being rather complicated, we must invoke the aid of the engraver once more. Fig. 8 gives you the method of making the pillars—A being the brass rod, B the screw and C the nut, the hole to admit screw to fasten the pillar to the base is made at the end D.

Fig. 9 is the brass spring with slit, A, to slip over the screw of Fig. 8, and the copper spring soldered to one side, at the end, at the point B. Now we slip the brass spring over the screw, the screw coming through the slit, and screw down the nut C. We thus have two springs supported at the ends on pillars at a height of 1 inch and ½ inch from the base respectively. Of course, both the pillars and springs are treated alike, but in the case of the tallest the copper is on the under side, and in the other on the upper side.

Now we go back to the armature, on the axle of which you will remember that I told you to fix a small roller of wood. This is only ¾ inch long and ½ inch in diameter, and is fixed firmly to the axle so as to revolve along with the armature. This roller is soaked in melted paraffin wax for an hour or two before fixing on, or boiled in it for some time, so that it may permeate the wood. The roller can easily be turned (of boxwood, preferably) if you are possessed of a lathe, but if you have none, go to the nearest photographer (or, preferably, a dealer in photographic apparatus), and from him you can buy for 3 cents a roller long enough to cut dozens for dynamos—they are what sensitized paper is sold rolled on.

The roller having been provided, take a piece of brass tube exactly so large inside that the roller will fit tightly into it, and cut off a piece the same length as the roller, or, if anything a trifle shorter. You have now to cut, with a saw or otherwise, two diagonal lines in this tube lengthwise, so that the tube is thereby divided into two pieces. Having done this the brass is replaced on the roller and fastened by minute screws, or “Prout’s elastic glue,” to each side of it, so that the roller becomes practically one of brass, with two slits in it. The screws must not project above the brass, but must be well sunk into it, so as to leave the surface smooth: and care must be taken that the screws do not touch both pieces of brass by going right through the roller—they must be very short. The object of cutting the slits in a diagonal direction is that the springs when pressing above and below the roller (see Fig. 10) shall not leave one half of the commutator before resting on the other part. If they do so the commutator will “spark” badly, which injures the fittings, and less current is obtained. Both slits are to be equidistant, and both inclined in the same direction. The roller is fixed on the axle in such a position that the middles of the lines of division are exactly in a line with the middle of the groove of the armature. When all this has been accomplished you will obviously have two conducting surfaces, each reaching over half the cylinder, separated by a small distance at top and bottom, the paraffined wood, of course, being a non-conductor of electricity. The brass tube must be made to fit smoothly round the wood, the surface being free from any irregularities, so that the contact with the springs at the sides may be as perfect as possible. Care must be taken that the brass is really separate all down on both sides. It is a good plan to fasten small splinters of paraffined wood in the slits to make sure.

This having been done, the wire from one end of the coil of the armature must be soldered to one of the semi-circumferences (if I may coin a word) of brass on the wooden roller, and the wire from the other end of the coil to the other semi-circumference. This is done at the end or underneath, not at the top, or it will make the surface rough, and we want it to be as smooth as it can possibly be. The wire must be quite tight up to the end soldered on; there must be no loops, or it will catch in something and be torn off when it comes to revolve.

The brass pillars supporting the springs have now to be inserted in the base, at such a distance, one on each side of the roller covered with brass, that the copper springs at the end of the brass ones are exactly one over and one under the brass roller. Of course, if they are put in a line with it, the springs can easily be shifted to the right position by slipping the slits over the screws of the pillars, and screwing down the nuts lightly when they come to the right place. This is very difficult to make intelligible, and I give another illustration of the relative positions of the parts of the commutator which I hope will make all clear. The pillars P P—which were put together as shown in Figs. 8 and 9—are fixed at such distances on opposite sides of the roller R that the springs S S are continually in contact with the brass semi-circumferences, first one and then the other as the armature revolves.

We are now within sight of the end of our task, and to guide off the current that we are going to produce we must screw in two binding-screws at opposite corners of the same end of the base (the end at which the commutator is). The ends of the wire from the magnet are to be brought down through the base and joined to the under part of these binding-screws. Placing the base so that the commutator end of the armature, and not the pulley end, is next to you, the wire from the inner coil of the magnet goes to the binding-screw on your left hand, and that from the outer coil to that on your right hand. The magnet should be wound and placed in such a position that these ends are respectively on the left and right, and then they have only to be joined to the binding-screws in front of them.

But before connecting these wires up, it is necessary to give an initial magnetism to the magnet, which at present has not been magnetized at all! To do this we must make use of another dynamo or a battery and connect the wires coming from the magnet-coil to the terminals of the battery. This having been done, the magnet will attract iron filings or needles, etc., and this shows that it has really become a magnet. Two cells of the chloride battery will be enough to magnetize it as much as it can be magnetized, and enough will remain when the battery is disconnected to start the action when the armature is revolved. Two or three minutes is long enough to connect with the battery.

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.