We can now proceed to fasten all the parts together. We must have a
piece of hard wood for the base, about 3½ inches by 3 inches and 3/8
inch thick. On this the magnet has to be fastened by its support being
screwed firmly down. In front of it the armature has to be fastened at
such a height as to be exactly in front of the poles of the magnet.
The relative positions of the parts are shown in Fig. 5, so I do not
think a detailed account of their exact positions on the base is at all
necessary.
There is, however, one piece of the mechanism in the figure to which I
have introduced you, this is the contact-screw shown at C. To make this
we take a piece of brass about 1½ inch long, ½ inch broad, and rather
less than 1/8 inch thick, and bend it at right angles, so that one leg
is one inch long and the other ½ inch. Now in the part that is ½ inch
have to be drilled three holes to fasten it with nails or screws to the
base. The other part, one inch long, will then stand erect, but before
fastening it in its place we put it to stand in front of the magnet and
mark a point which is exactly on a level with the piece of platinum
foil on the spring, when the spring and magnet are fixed in position.
A hole has now to be drilled through that point and tapped to admit a
brass screw with a milled head, and fix the piece in which the screw
works to the front hole, so that the screw will work through it.
[Illustration: FIG. 5.--INTERIOR MECHANISM OF DRUM COMPLETE.]
The point of the screw has now to be cut off and a very small piece of
platinum wire fixed at the end. This wire will now come in contact
with the platinum foil on the spring, when the brass support is fixed
in a certain position on the base, and it is now to be fixed in that
position with screws or nails. It should be so fixed that when the
screw is turned till it is nearly out of its hole the wire is just out
of contact with the platinum foil on the spring. It is now evident that
by turning the screw one way you make the spring vibrate more rapidly,
and by turning it the other way its efforts are relaxed.
The contact-breaker screw having been fixed in its place, and the
support of the spring also fixed as at T in the diagram (Fig. 5)--by
screws through the base into the iron, if it is made of iron, or by
nails or screws through it into the base if of wood--all the parts are
now together, and all that remains to be done is to make the necessary
connections. One wire that comes from the magnet is to be joined
(soldered, if possible,) to the spring at H in the picture; the other
wire is left loose. To the brass support of the contact screw we solder
another piece of wire. Now this piece of wire is connected with the
zinc of the battery and the other (coming from the coil of the magnet)
with the carbon of the battery. What happens?
The electricity passes along the wire X, we will say, and round the
magnet coils, thus turning the cores into magnets. It then goes down
the other wire to H, up the brass spring, along the screw, and down
by the brass support to the other wire, by which it returns to the
battery. That is to say, it _would_ do all this if the armature stood
still, but, of course, when the cores become magnets they attract the
armature, which instantly moves towards them; this breaks the circuit,
the spring moving off the platinum point of the screw, and the armature
springs back again, which makes the circuit complete and the magnet
attracts it again, and so on. The object of the spring is to get a good
deal of vibration, and it and the screw should be so adjusted that
although the armature is close enough to the magnet to make it certain
to "go off" directly it is meant to do so, yet there may be as much
scope for the spring to work with elasticity as possible.
We have now completed the electrical part of the business, but a
slightly necessary part of the apparatus has yet to be obtained--viz.,
the drum. You can easily make a drum if you like, by taking a broad
piece of tin, twisting it round to form a hoop, and covering the ends
with parchment strained tightly over them. However, I should certainly
not do so, for there can hardly be any spot, I should think, which
boasts of a toy-shop at all, where drums cannot be procured! For
twenty-five cents you can get a very superior drum, just about the
right size; if you like to get a bigger one and make the mechanical
part bigger, you will, of course, be rewarded by more noise.
Now, suppose you have got a 25-cent toy-drum, you must proceed to take
off one end. If you look at the construction of the drum you will find
(at least it is the case with my own, and I have not seen any that are
differently made) that by cutting one of the double strings that fasten
the wood hoops at the top and bottom together, and then loosening all
the other strings with your fingers, the wooden hoop at one end will
come right off, if the nails fastening the ends together are taken out,
and that then the inner hoop on which the parchment is stretched will
also come off and leave that side of the drum open.
Now, this is simply grand for our purpose, for when we have arranged
our little dodges inside the drum, we can put on all the hoops again,
replace the one double string, and no one will be an atom the wiser. If
you could get off the side without breaking any strings it would save
the trouble of replacing any, but I am afraid this is hardly possible.
However, off comes the side of our drum, and what is to be done next?
Well, the "beater" must be put bodily inside the drum, just so close
to the parchment side that was taken off that the wooden head of the
drumstick touches it when attracted by the magnet. You can easily find
the right place in actual practice by setting the beater going and
finding the spot inside the drum where it kicks up the worst racket
when working. It must not be too close or it will hinder the vibration,
and we want the hammer to go off instanter when required. The beater is
fixed to the side of the drum with its side marked Z in the figure (5)
downwards. It is easily fastened there by making two holes in the wood
(in the thickness of it), and two corresponding holes in the metal side
of the drum, and then screwing it down in its proper place.
Two holes are to be made in the side of the drum and two ornamental
bits of silk-covered flexible copper conductor let through. They can be
secured by simply tying knots inside the drum, and the copper ends are
now to be fastened, one to the wire X and the other to the wire K from
the contact screw support. Having done all this and made sure that the
beater works when the ends of the flexible cord outside the drum are
connected with the battery, we seal up our drum again, and that is then
concluded.
Now as to fixing it up, I think I may fairly assume that you know
how to make it work by an ordinary battery and a "press." It is only
necessary to run a double wire from battery to press and from press to
drum, one wire of the double conductor being fastened to the carbon
end of the battery and the other to the zinc end, and the other end
of one wire to one of the wires coming from the drum. The other wire
coming from the drum is then joined to the bottom conductor of the
press, and the upper conductor of the press is joined to the other wire
of the double conductor that goes to the battery. It is all very easy
to understand if you follow the course of the current and consider that
it has to pass through the drum and the press when the latter is pushed
down, and be stopped when it is left to spring up again.
But the more magical arrangement can be made with the drum, and I
think it is well worth while to do it, if merely for the fun of
mystifying people. The drum is going to be suspended by the flexible
cords; therefore, let them be the same length, and cutting off all
the coverings at the end of each, fasten a brass "eye" to the copper,
twisting the wire well round the bottom of the eye. Now wind silk of
the same color as the rest all round the join, so that the connection
of wire and eye is completely hidden, and the eye appears merely
fastened to the flexible cord as a means of suspending the drum. Now we
want to construct a hook from which the drum can be hung.