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 ⅞ 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 ⅛ 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.
Fig. 8.—Pillar of Commutator.
A, Brass rod. B, Screw inserted at end. C, Nut fitting screw B. D, Hole for screw to fix to base.
To make this necessary adjunct to the dynamo, take a circular bar of brass rod about ⅜ 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 ⅜ 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 ⅛ 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.—Brass Spring of Commutator.
A, Slit to fix over screw, B, in Fig. 8. The shaded part represents the copper spring, soldered at B.
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.
Fig. 10.—Section of Commutator Put Together.
P P, Pillars supporting springs, S S, which bear respectively on the upper and under sides of the roller, which is covered with brass except for the slits shown in the diagram.
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.