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HOW TO MAKE ELECTRICAL MACHINES.
Containing full directions for making electrical
machines, induction coils, dynamos,
and many novel toys to be
worked by electricity.
by
R. A. R. BENNETT.
Fully Illustrated.
New York
Frank Tousey, Publisher
24 Union Square
Entered according to Act of Congress, in the year 1900, by
Frank Tousey,
in the Office of the Librarian of Congress at
Washington, D. C.
How to Make Electrical Machines.
How to Make a Simple Electrical Machine.
I propose to describe a method of making an electrical machine of small
dimensions, but capable of performing all the experiments that are
likely to be required of it.
[Illustration: FIG 1.--BACK OF RUBBER, SHOWING POSITION OF HOLE.]
For the stand of the machine take a piece of wood (deal will do, but
mahogany would be preferable) 14 inches in length, 8 inches in breadth,
and 5/8 inch in thickness. To the bottom of this fasten two more pieces
of the same wood, 1¼ inches broad, 8 inches long, and 5/8 inch in
thickness at opposite ends, so that the edges are flush with the board.
This forms our stand, on which we now proceed to erect the machine.
Take another piece of the same wood, 7 inches long by 2½ broad, and 7/8
inch thick and fasten it firmly by four screws at the ends to the base
board at a distance of half an inch from one end of its length and in
the center of its breadth.
We now take two pieces of wood 14½ inches long by 2¼ inches broad and
½ inch thick, and fasten them upright to the opposite sides in the
center of the piece just fixed to the board. They must be fixed very
firmly to it with several screws, as they have to bear a severe strain
while the machine is worked.
[Illustration: FIG. 2. DIAGRAM SHOWING POSITION OF PLATE AND RUBBERS.]
If the reader can _dovetail_ the ends into the cross board they will
be held much more firmly. At the top of these pieces another piece of
wood, 3¼ inches square by 3/8 inch thick, is fastened by screws into
the upright pieces, so as to hold all firmly together.
[Illustration: FIG. 3.--SECTIONAL DIAGRAM OF CONDUCTOR.]
The framework of the machine is now complete, and we have to provide
the glass plate from which the electricity is to be produced. As we
cannot make this we must apply to an electrician for it. This is 10
inches in diameter. If the maker is good at, and has appliances for,
working in brass on a small scale, he can make the axle himself by
taking a piece of brass rod ¼ inch in diameter and 3 inches long and
fastening the glass plate in the center.
This can be done by providing two circular caps of brass one and
one-half inches in diameter (the side of which next the glass must
be covered with cloth to prevent cracking the glass), and fastening
one by solder or otherwise, on one side of the plate, the other being
arranged to screw up tightly on the other side, by having the brass
turned into a screw, and the center hole of the cap made with a flange
to fit it. If this is beyond the reader, he must be contented with a
less elaborate axle of wood instead of brass, and two wooden caps which
can be firmly fastened to the axle and glued to the opposite sides of
the glass plate with Prout's elastic glue, which can be bought from any
harnessmaker.
If this is used care must be taken in warming the glass not to render
the glue too soft to hold it firmly when turned by the handle. To turn
the axle it must be provided with a handle of wood, in the case of
the wooden axle, or, in the case of the brass one, a handle is made
by turning the projecting end of the axle into a screw and fitting to
it a piece of flat brass three and one-half inches by one-half inch
by one-eighth inch, this latter piece having another piece of brass
rod three and one-half inches long fixed to the other end, on which a
wooden handle is fixed (by a cap fastened at the end of the rod) so as
to turn freely.
[Illustration: FIG. 4.--SHAPE OF BRASS ROD TO COLLECT THE ELECTRICITY.]
The glass plate having been thus mounted, we must turn our attention to
the rubbers which generate the electricity on the plate. To make these
take four pieces of wood 3 inches by 2½ inches by 3/8 inch, and on one
side of them fix pieces of thick flannel (which you can get nearly ¼
inch in thickness) of the same size, and cover these over with black
silk, gluing it down lightly to the wood, so as to form a thick cushion
on one side of it. These four cushions have now to be fixed so as to
be firmly pressed against the glass plate while it turns. This can be
done by fastening them at the backs by screws to the upright pieces
supporting the plate, or by gluing four small pieces of wood about 1/8
inch thick, and square in shape, to the inside of the supports. The
rubbers then have four holes cut in their backs to fit these pieces of
wood, on which they slide when placed on the side of the glass, and are
thus held firmly in position. Fig. 1 shows the position of the holes
on the backs of the rubbers. The latter plan is the best for fastening
the rubbers, as it allows them to be removed at any time for warming (a
very essential point) or spreading fresh amalgam on them. Fig. 2 shows
the position of the plate and rubbers when in their places.
[Illustration: FIG. 5.--SECTIONAL DIAGRAM SHOWING POSITION OF COLLECTORS
AND PLATE.]
We now have the means for procuring electricity, but the method of
collecting it has yet to be provided. To make this a conductor must be
formed by cutting a piece of wood to the shape of Fig. 3. It should be
about 6 inches from end to end, and must be carefully rounded so that
no projections are left on it. It must then be covered carefully with
tinfoil (which can be obtained from a chemist), the tinfoil being glued
down as smoothly as possible. From the end of this conductor a piece of
brass rod should be fixed, shaped as shown in Fig. 4. A piece about 12
inches long will be wanted. This must be bent at the ends, so that when
the conductor is mounted on a stand consisting of a piece of glass rod
6½ inches high, fixed to the center of the stand (that is 5 inches from
the opposite end to that at which the supports are), the glass plate
revolves between two surfaces of the brass rod. Fig. 5 explains the
arrangement, which is somewhat complicated to describe. The glass rod
should be about 7 inches long, to allow of half an inch being inserted
into a hole in the center of the conductor, which is thus supported 6½
inches high from the stand.
[Illustration: FIG. 6.--THE MACHINE WHEN FINISHED.]
It now only remains to fasten several small pieces of brass wire about
a quarter of an inch long, filed to a point, to the sides of the rod
nearest the glass plate, as shown in Fig. 6, so that the plate revolves
between a double row of points, which can be done with solder, and the
machine is complete. The conductor can further be improved by inserting
at the opposite end a small piece of brass rod two and a half inches
high, surmounted by a brass ball, which is useful in some experiments.
Care must be taken that the tinfoil of the conductor overlaps the brass
rod at either end, and thus forms a metallic connection. If this is
not done the conductor will not become charged sufficiently. If the
conductor can be made of brass it will work better still, as a metallic
connection is then insured. The conductor can be fastened to the glass
rod on which it is supported by "Prout's elastic glue," or other
cement, a hole being made in the center of the bottom of the conductor,
and another in the stand of the machine for opposite ends of the glass
rod.
The machine having been constructed, a few words will be useful in
how to work it. Warmth and dryness are, above all things, essential.
If the air of the room is damp it will be nearly impossible to obtain
any result. Before working, the glass plate must be thoroughly warmed,
taking care not to crack it, by being placed endwise before a good
fire. A silk handkerchief is a useful adjunct to the machine.
The glass plate should be wiped quite free from dirt, and the glass
support of the conductor must also be wiped, the handkerchief being
made very hot. The rubbers must be taken off (if constructed so as to
be movable, as described), and placed before the fire till quite hot.
Their powers may be enormously increased by covering them with amalgam,
as sold in the electrical shops, but a far better plan is to cover the
cushions with tinfoil, which can be glued right round the rubbers and
over the backs. This will need renewing at intervals, as the plate in
turning wears it out.
Now, when the rubbers are quite hot and all the glass of the machine is
dry and hot (this is necessary, because, if damp, the electricity would
escape without producing any effect), the rubbers are put into their
proper places on each side of the glass, and on turning the handle
(which will be rendered easier if the machine is firmly clamped to the
table) and approaching the knuckle to the conductor, a succession of
brilliant sparks will be emitted from the conductor. If this does not
happen either the glass or some part of the machine is damp, or the
machine is not put together quite correctly, and must be examined to
find out the fault.
A machine of the size described should give a spark an inch long when
working properly. A great number of experiments may be performed with
this machine with apparatus capable of being made at home. I give
a final illustration (Fig. 6) to show how the machine looks when
completed.
How to Make an Induction Coil.
[Illustration]
To most boys electricity offers many attractions, and as I have
recently constructed an induction coil out of materials which are cheap
and easily obtained, I think I shall confer a benefit on many readers
if I give them a short description of how this was accomplished,
so that if like-minded they can proceed in the same way. Induction
coils may be used for medical and scientific purposes as well as for
amusement, so that a good deal of work comes within their scope. An
"induction coil" is composed principally of two portions--one is the
"primary" coil, the other the "secondary." It is the secondary coil
that gives the spark, and on the length of this depends the power of
the coil; in some instruments for scientific purposes it is composed
of a wire nearly 300 miles long--but we are not going to soar to such
heights as that!
To make the coil itself you want an ounce of "No. 24" cotton-covered
wire, and two or three ounces of "No. 36." This can be bought from
an electrical supply dealer. If you are very ambitious, silk-covered
wire can be used; this gives better effect, the insulation being more
complete.
[Illustration: FIG. 1.--FRONT DISC.]
[Illustration: FIG. 2.--BACK DISC.]
To form the groundwork of the apparatus take a piece of mahogany
about half an inch in thickness and polish it up to look ornamental;
it should be about 4 inches by 6 inches for the sized coil I am
describing. We now take another piece of mahogany about ¼ inch thick,
and from it cut two circular pieces about 1½ inch in circumference;
these are to form the ends of the coil; they must each have a hole
3/8 inch in diameter drilled in the center for the ends of the core
to pass through. In one of them, which is to form the coil, two much
smaller holes are drilled with a small bradawl to allow the ends of the
primary coil to pass through (Fig. 1); in the other two similar holes
are drilled further from the center for the ends of the secondary coil
(Fig. 2). This having been done, we proceed to form the _core_, and
this being the most important part of the instrument, it must be made
with great care. Take a length of fine iron wire (annealed) and cut it
into pieces 2½ inches long.
Now take a brass tube of the same size internally as the center holes
in the ends of the coil were made (3/8 inch) and push as many pieces
of wire into it as are required to pack it as full as it will hold.
The next thing to do is to take another piece of wire and wind it as
tightly as possible round the ends of the wires, pulling them gradually
out of the tube as you wind, until they are entirely out, by which time
a compact bundle of iron wire will have been formed. Now file the ends
of the core thus formed, quite smooth, with a fine file, and drop the
whole of it, wire and all, into the hottest part of a fire. Leave it
there till it is bright red hot all through, and then rake it out and
bury it completely in the ashes under the grate. If this can be done
over night, and the coil left to get cold as the fire goes out, instead
of being placed in the ashes, so much the better, as the object is to
cool it as gradually, and thus make it as soft as possible.
[Illustration: FIG. 3.--CORE AND DISCS.]
When it has become perfectly cold take some paraffin wax and melt it
in a dish. When thoroughly melted, heat the core again gently, and put
it into the melted wax. Leave it there for a short time till it is
thoroughly saturated with the melted wax, then take it out and hold it
above the dish to let the melted paraffin run back into it. When cold
you may remove the binding wire, and the wax will be found to hold all
the pieces together in a solid lump. The two pieces of wood must now be
fixed one at each end of the core (the holes being the same size as the
bore of the brass tube, the core should fit into them quite tight), one
of them (the front) being pushed a little distance over the core, so as
to leave about ¼ of an inch of the core projecting from it, the other
one only being pushed on sufficiently far to make the end of the coil
flush with the wood (Fig. 3).
Take a sheet of thin notepaper and cut a piece exactly the width of the
coil, and long enough to pass twice round it. Wind it tightly round,
and fasten it, if necessary, with a little paraffin. Now the wire has
to be wound on over the paper, the thickest first, to form the primary
coil. Pass about three inches of one end of it through one of the holes
in the disc forming the front of the coil, and then wind it evenly on
the core, taking care that each coil is separate from its neighbor, and
that no two coils fall one upon the other.
When the wire has reached the other end of the core, wind it back again
over the first layer till it reaches the end it came in at, then pass
it through the other hole and cut it off about three inches from the
hole; the wire cut off will be wanted for other purposes. The secondary
coil has now to be wound over the primary, first of all saturating the
cotton with which the latter is covered by pouring melted paraffin
over it with a spoon. All the secondary wire will be wanted; it must
be wound layer above layer exactly as the primary was, first passing
about three inches of the end through one of the holes in the disc at
the back of the core. A thickness of notepaper should be put on between
the primary and secondary coils. Everything depends on the complete
insulation of one coil from another, and this is accomplished by means
of the notepaper and cotton, saturated with melted wax in subsequent
operations. When the whole of the secondary wire is wound on except
about three inches, pass the end through the other hole in the disc.
In order to make sure that the wire has not been broken in the
winding, which would entirely destroy the action of the instrument,
the two ends of the coils should be joined separately with a battery
and galvanometer. If the needle is deflected on joining the circuit
the wire is all right. This is rather important, as it is extremely
vexatious, when all the different parts have been adjusted, to find
that the coil will not work owing to a fracture of the wire, which
necessitates the whole coil being unwound before it can be discovered.
If the galvanometer is not at hand we must take our chance; the
greatest possible care must be taken in winding the secondary wire,
as this thin wire is extremely brittle. The insulation must now be
improved by plunging the whole coil into a deep vessel large enough
to contain it, which is full of melted paraffin. This must be placed
near the fire, so as to keep the wax melted, and the coils must be left
in it to soak for an hour or two. When the paraffin has thoroughly
permeated through it it can be taken out and held above the vessel to
drain. If all the wax does not run off the ends they can be scraped
afterward, taking care not to cut the wires. The appearance of the coil
is vastly improved by a strip of velvet cut the right width, which can
be drawn tightly and sewn in position; or the coil may be covered with
a varnish made by dissolving red sealing-wax in spirits of wine by the
aid of a gentle heat. The coil part of the instrument is now complete,
and ready to be affixed to the base-board by means of two small screws
passing through it into the discs when placed in the proper position
(see Fig. 6.)
We now approach a very important and rather intricate piece of
workmanship. It is necessary, in order that shocks should be obtained
from the coil, that the current in the primary wire should be stopped
and started again at the rate of several hundred times per minute, and
the more quickly the contact between the battery wire and the primary
coil is made and unmade the more powerful the shock. In order to
accomplish this a "contact-breaker" becomes necessary, the method of
making which is as follows:
[Illustration: FIG. 4.--HAMMER OF CONTACT-BREAKER.]
A piece of sheet brass is taken 1½ inches long by about 3/8 of an inch
at one end, gradually tapered up till it comes to a point about 1/8
of an inch broad at the other; it must be very thin, and must act as
a spring when fastened tightly at one end. A small piece of soft iron
is soldered to the small end of this to be attracted by the core when
working. The next thing is to fasten a small piece of platinum foil
about ¼ of an inch square on the opposite side of the brass to the
soft iron, and a little below it (Fig. 4). This is rather a difficult
operation, as it is such a small object to solder, and the best way is
to get it done by a tinsmith, unless you are skilled in the use of the
soldering bit.
[Illustration: FIG. 5.--SCREW OF CONTACT-BREAKER.]
[Illustration: FIG. 6.--PLAN OF COIL COMPLETE.]
A narrow strip of stout brass is now taken and bent at right angles
near one end, so that when screwed down to the base-board by holes in
the smallest leg the longest leg will stand upright. Stand it up on
the base in front of the coil and note a point on the strip exactly
opposite the core. Make a hole through this point large enough to admit
a small screw used on paper fasteners. Now take the flange part of the
paper-fastener and solder it to the back of the brass strip, so that
the screw will work through both (Fig. 5). This is done to avoid the
trouble of making a flange in the strip, but if you _can_ do this, so
much the better.
Now, the coil having been fastened to the base by fine screws through
it into the ends of the reel, nearly in the center of the base, we
must find a place on the base in a straight line with the end of the
core (as at C, Fig. 6), and here we fasten another piece of bent brass
similar to the last. The end of the contact breaker is now soldered
to this brass strip in such a way that the piece of soft iron at the
other end is exactly opposite the core and about 1/16 inch distant from
it. The screw of the paper fastener must now be tipped with platinum
by cutting off the end and drilling a fine hole in it, in which hole a
small piece of platinum wire can be soldered. The amount of wire and
foil required, although very minute, will cost you about twenty-five
cents, platinum being a very expensive substance. It can be bought from
a chemist or electrician.
The screw having been prepared in this way, we must next fasten the
brass strip to which the flange is soldered upright on the base, so
that the platinum point of the screw, when inserted, will just come in
contact with the square of foil on the spring. By turning the head of
the screw the soft iron can thus be forced nearer the core, and the
rapidity of its vibration is thus controlled. The coil is now complete,
except the connections, which are made (preferably underneath the base
by letting the wires through) by joining the ends of the thin wire
to two "binding screws," which are made for this purpose and can be
obtained from the dealer. One end of the thick wire of the coil is
fastened to the strip of brass supporting the contact-breaker, the
other end is fastened to a binding-screw on one side of the base--the
strip of brass supporting the screw being connected by a wire with
another binding-screw on the other side. This sounds rather intricate,
but will easily be understood if we consider that the current from
the battery when the wires are connected with the binding-screw
must pass through the brass strip to the screw, thence through the
contact-breaker to the coil, and, having passed round the coil, back to
the battery through the binding-screw attached to the other end of the
wire. (See Fig. 6.)
It is now evident that when the contact-breaker is in contact with the
screw a current will pass through the primary coil, and will cause
the soft iron core to become a magnet and thus attract the soft iron.
When this moves towards the magnet, contact is broken and the core is
instantly demagnetized, so that the spring flies back and contact is
made again. The screw is adjusted so that the contact is broken just
as the soft iron touches the core. When the battery is joined on,
the contact-breaker will fly backwards and forwards with tremendous
speed, making a loud, buzzing noise, while brilliant sparks will appear
between the platinum wire and foil.
In order to feel the effect of the shock, two handles will be required;
these can be made by simply bending two pieces of tin about two inches
by four inches round a ruler and neatly soldering the joins. A wire is
now fastened to the end of each tube, the other ends being inserted in
the binding screws connected with the thin wire of the secondary coil,
which are at the opposite corners of the base to those which are joined
to the ends of the primary coil. When the coil is buzzing, if these
handles are tightly held, a powerful shock will be felt, in fact, a
weak battery only should be used with the coil of the dimensions given,
or it may be impossible to release the handles, and this is too strong
to be pleasant.
The current can be regulated by means of a "regulating tube," that is
simply a brass tube which is made to slip over the core between it and
the primary coil; the farther the tube is pushed over the core, the
less powerful the shock. The dimensions of the coil being the same, a
little ingenuity will enable any one to affix a regulating-tube. I will
only say that instead of winding the coil direct on the core a tube
of brown paper is formed a little larger than the core, and on this
the wire is wound. Between this tube and the core the brass tube is
arranged to slip in and out, the hole in the end of the reel farthest
from the contact-breaker being made larger for its accommodation.
This concludes my description of the coil, but perhaps a few hints as
to suitable batteries may be useful. If a strong battery which will
only work the coil for a short time is required, the bottle bichromate
is a good one. It can be bought from a dealer, or one can be made in
a simple form by taking a jar and filling it with a strong solution
of bichromate of potassium, to which a little sulphuric acid has been
added. Take two pieces of gas carbon and three pieces of sheet zinc,
both cut to the right size to dip in the solution to the bottom of the
jar.
At the top of the zincs and carbons bore small holes, and below these
place narrow strips of wood to keep them apart when in use; these must
be long enough to reach across the top of the jar when the zincs and
carbons are in the solution.
Arrange them thus: zinc, wood, carbon, wood, zinc, wood, carbon, wood,
zinc; bind them lightly together by means of two more pieces of wood
placed outside the outer zincs, and the whole tied together with
string. Connect the three zincs together with one piece of wire, and
the two carbons with another, taking care that the wire connecting the
zincs, does not come in contact with the wire connecting the carbons.
To one zinc attach a piece of covered wire, and to one carbon attach
another, these two wires are connected with the binding screws of the
primary coil. This battery is extremely strong, double as strong as the
bottle bichromates sold, as there are more zincs and carbons employed,
but it only lasts a short time before needing to be replenished.
Daniell's battery is a weaker form, but lasts much longer, say for two
or three hours in constant work. Take a deep jar and inside it place
a porous jar of earthenware, which the electrician will provide. Now
get a piece of sheet copper of the right size to go into the jar, and
bend it round so that the porous jar will go inside it. A piece of
sheet zinc will be wanted to go inside the porous jar. Both zinc and
copper must be high enough to reach the level of the solutions when the
jars are full. The porous jar is filled with dilute sulphuric acid, or
solution of common salt; the jar outside is filled with "_saturated_"
solution of sulphate of copper--that is, as strong as it can be made.
Lumps of sulphate of copper are kept in the outer cell, which will
keep the solution concentrated by slowly dissolving. Attach one wire
to the zinc and another to the copper, and when these 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 7/8 inch thick. This can be stained afterwards to
make it look nicer; it must be planed well and polished up quite smooth.
[Illustration: FIG. 1.--SECTIONAL DIAGRAM OF ONE SIDE OF MAGNET.]
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 1/8 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, 3/8 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.
[Illustration: FIG. 2.--MAGNET PUT TOGETHER.]
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.
[Illustration: FIG. 3.--ARMATURE OF DYNAMO.]
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.
[Illustration: FIG. 4.--SECTION OF END ARMATURE.]
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 1/8 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.
[Illustration: FIG. 5.--SUPPORT FOR PULLEY END OF AXLE.]
Returning to Fig. 1, we must see that the groove A, which forms
half the channel in which the armature is to revolve, is 7/8 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.
[Illustration: FIG. 6.--SUPPORT FOR COMMUTATOR END OF AXLE.]
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.
[Illustration: FIG. 7.--GROUND PLAN OF MAGNET AND ARMATURE WHEN PUT
TOGETHER.]
The brass of which the supports are made should be about 1/8 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.
[Illustration: FIG. 8.--PILLAR OF COMMUTATOR.]
To make this necessary adjunct to the dynamo, take a circular bar of
brass rod about 3/8 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 3/8 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 1/8 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.
[Illustration: FIG. 9.--BRASS SPRING OF COMMUTATOR.]
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.
[Illustration: FIG. 10.--SECTION OF COMMUTATOR PUT TOGETHER.]
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.
[Illustration: FIG. 11.--DYNAMO COMPLETE. GROUND PLAN.]
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.
[Illustration: FIG. 12.--HAND-WHEEL ARRANGEMENT FOR WORKING DYNAMO.]
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.
PART I.
THE ELECTRIC TRUMPET.
There are many toys which one meets with in the scientific stores,
the making of which for themselves would give great satisfaction to
enterprising devotees of the electrical art. They are for the most part
easily constructed, and a great deal of amusement can be derived from
them. I have my doubts whether the fathers and mothers of the amateur
electrician will thank me for introducing the subject of the present
article, but they must take comfort in the thought that if it works
well it shows real constructive power on the part of the maker.
For the benefit of those whose capability of working in metal is
limited, I am first going to describe the making of this remarkable
instrument in its simplest form--a form, in fact, so simple that any
one can make it and achieve success in a few hours.
First of all we want an old tooth-powder box. These are all made the
same size, and consequently it is unnecessary to give dimensions.
The top of the tooth-powder box is to be taken, and by means of a
fretsaw (this invaluable tool should be in the hands of every boy who
likes carpentering; there are many uses to which it can be put quite
different for what it is intended for) a circular hole is to be cut out
about 1/8 inch less than the inside--that is to say, a rim of about 1/8
inch is to project all around from the rim of the lid.
We now want what is known in photography as a "ferrotype" plate--_i.
e._, a piece of very thin sheet iron. Most dealers in photographic
goods will not sell less than four or five dozen of them, and this is
too many for us. A photographic friend will let us have one gratis, or
a professional photographer _may_ agree to part with one for five or
ten cents if he is attacked when in a good temper.
The ferrotype plate having been procured by some means or other, the
next thing is to cut from it a circle just small enough to go inside
the rim of the top of the tooth-powder box. You can mark out the circle
before cutting it by painting the top of the rim of the bottom of the
tooth-powder box with ink and pressing it down on the ferrotype plate,
when enough ink will come off to guide the scissors, and of course the
circle so cut will be the exact size required.
We now have to make the motive power of the machine, for there is
plenty of work done in it, though it only makes a noise--no one can
"make a noise in the world" without doing plenty of work! And to make
this we take a piece of soft iron rod about 1½ inch long and half an
inch in diameter, and cut two circles out of cardboard 1¾ inch in
diameter. The soft iron rod can be bought from any hardware store, and
it ought to be quite soft enough to work at once without doing anything
to it; if it is not, it must be heated red-hot in a good fire and left
among the coals over-night to get cool very gradually.
Personally I have always found that the ordinary bars of soft iron
bought from any hardware man are amply soft enough for any electrical
work.
You must get the hardware man to file the ends of your bar flat; if
they are not filed you will have to do it yourself, and a fine job it
is!
Now we go back to the circles of cardboard. A hole is to be cut in each
in the center exactly the size to admit the core of soft iron, then by
slipping the circles over the ends we get a reel. Now a hole has to be
made exactly in the center of the bottom of the tooth-powder box, and
exactly so large that the core of soft iron will fit tightly into it;
you can do this again with the fretsaw, the wood of which tooth-powder
boxes are made is delightfully easy to cut.
Now comes the adjustment of the reel. You must put the circles on the
core, and putting one end of the latter through the hole at the bottom
of the box you must push the iron through until the top is exactly
flush with the top of the rim of the side of the box. One of your
circles will now be much further on the core than the other, and the
one at the end that is not pushed through the hole must be adjusted
close to the edge, leaving about 1/16 of the core projecting, so that
we have now a reel formed at one end of the core, and held in position
by the bottom of the box. The more stiffly the core fits the hole the
better, and if it has to be hammered into its place, better still, only
take care not to split the wood of the bottom of the box.
The circles, being now in their right places, must not be moved again,
but the roller has to be wound with wire, for which purpose the core
will have to come out of the box temporarily. Before beginning to wind
the wire, get some thin paper (French note-paper is best), and wind a
piece round and round the core between the circles, fastening it and
the circles at its ends to the core by means of a small quantity of
mucilage.
We now have to wind the wire on to the roller. The more wire the
stronger the magnet will be, but sufficient will be about two ounces.
You can get the wire at most hardware stores for fifteen cents an
ounce. It is generally cotton-covered, of light green color; medium
thickness should be used, not too fine, as this offers too much
resistance to the current, and not too coarse, or it will fill the reel
too soon.
We begin by making a hole near the core in the circle which is furthest
on it, and push one end of the wire through a hole from the inside of
the reel. About three inches should be pushed through to allow for
future manipulation, and the wire is now to be wound tightly over the
paper covering the core in even coils, layer on layer, till the reel
is nearly full and we have arrived at within about three inches of the
other end of the wire. This is now to be passed through another hole in
the same circle as before, which hole will of course be further from
the center than the first. The magnet will be much stronger if two or
three folds of paper are wrapped round it between each layer of wire.
The coil is now constructed, and can be replaced in the tooth-powder
box, passing the ends of the wire through two holes in the side or
bottom made to receive them. Before leaving this part of the instrument
I may remark that care must be taken that the covering of wire is quite
continuous throughout, and has not got rubbed off at any points; if it
has, you must wind fine silk over it to cover it up again. Should there
be a break anywhere in the wire you must carefully scrape the wire off
the two ends and twist the wires firmly together, if possible soldering
them together and then wind fine silk over the join.
It is not necessary in this machine to soak the coil in melted
paraffin, but might improve the insulation if the cover of the wire
is thin. Only if there is a join and you have twisted, not soldered
the wires together, you must not soak the coil in wax, or the melted
wax gets between the ends of the wires and stops the current (this of
course applies to all electro-magnets and should be remembered as a
possible cause of failure.)
The core having been pushed through the hole again, up to the circle
of cardboard, the ferrotype plate is placed in the top of the box, and
the box is shut up. Now the ferrotype plate must be exactly free of the
end of the core and that is all. You can test this by tapping it. If
it vibrates in and out, it is all right; if the end of the core is too
tightly pressed against it, there will be no possibility of moving the
center in and out, and the core must be driven further through the hole
till it is just free of the ferrotype plate.
[Illustration: FIG. 1.--SHAPE OF PLATINUM FOIL, P, FASTENED TO
FERROTYPE PLATE, F.]
Now comes another part of the instrument, viz., the contact-breaker.
The following is as good a way of arranging it as any: Take a piece
of sheet brass the exact length of the diameter of the top of the
tooth-powder box and about half inch wide, and in the middle of it bore
a hole which will admit a brass screw--with a milled head preferably.
The screw should fit tightly into the hole, so as to screw easily up
and down when turned. To the end of the screw, which is cut off flat,
is soldered a short piece of platinum wire, inserted in a hole in the
end of the screw made to receive it; it can be fastened by any other
means, as long as it will screw up and down and is in contact with the
brass screw. Adjust the screw so that the platinum point is within
a minute distance of the ferrotype plate when the brass support is
screwed down at the ends to the side of the box lid, and screw it down
with small screws firmly in its position.
[Illustration: FIG 2.--SECTION OF SIMPLE ELECTRIC TRUMPET SHOWING
DETAILS OF VARIOUS PARTS.]
Before this is done, however, a thin strip of platinum foil should be
soldered to the upper surface of the ferrotype plate, or otherwise
fastened to it--elastic glue will answer--this strip terminating in the
center, and reaching to the edge of the plate, leaving a short piece
over. A very thin strip will be enough, of the shape of P in Fig. 1.
Now the ferrotype plate is to be placed in position again (the side
of which the platinum foil is fastened being outwards, and the end of
the foil going down between the edge of the ferrotype plate and the
wood into the inside of the box), and the end of the wire from the coil
which was left inside the box is to be securely fastened, either by
soldering or otherwise, to the end of the platinum foil which was left
loose, so as to be in metallic connection with it. A wire can now be
twisted round or soldered to the screw with the platinum point, and the
instrument is complete.
It has taken some space to describe, but I made my own in about half an
hour. Fig. 2 gives a general view of the parts put together.
The lid of the box should be tightly fastened down by four small
screws, two of which may be those which fasten on the brass strip
holding the screw.
Now to consider its action. The wire I in Fig. 2 is connected to one
wire of the battery, and the wire G to the other. The current then
starts from the battery, round the coil B, converting the core into a
magnet, and up the wire H to the platinum foil P, along the platinum
foil, which was fastened to the upper side of the ferrotype plate F,
to the platinum wire which tips the screw C. It then goes up the screw
C, along the brass piece E, which is fastened to the box by screws, as
shown in the figure, to the wire G, and so back to the battery by the
other wire.
The screw C must be therefore screwed down till the platinum wire at
its tip is just in contact with the foil on the ferrotype plate. Now
of course when the current goes round the coil, and thus converts the
soft iron into an electro-magnet, the latter instantly attracts the
ferrotype plate which is immediately above it. But the latter moving
its center near the core, the platinum foil which is attached to it
is thereby moved out of contact with the wire on the screw C, and the
current is instantly stopped. Thereupon the attraction of the magnet
ceases, and the ferrotype plate flies back to its former position and
so joins the platinum wire and foil, and starts the current again, and
the former process is repeated. The ferrotype plate therefore vibrates
with tremendous rapidity between the core and the platinum screw. Now
the vibrating armature of an ordinary coil makes quite a hum when hard
at work, but of course a large plate such as this makes a much louder
noise, consequently you will hear a ferocious buzzing like an army of
millions of bees let loose from a hive, and on screwing the screw C up
or down till you get to the correct point you will get a shrill note
very like a penny whistle. If screwed up the vibrations are slower,
and a deeper note is produced; if screwed down the vibrations are more
rapid and a higher note is sounded. Therefore you can amuse yourself by
screwing it rapidly up and down, or adjusting it by pressing the brass
piece with your finger, and a little practice will enable you to bring
out a sort of tune produced by electricity!
When you have become tired of jingling out your tune you can fix the
electric trumpet up in a permanent position, adjusting the wires from
the battery so as to pass through an ordinary "press" which may be in
another room. The trumpet will then begin buzzing or hooting whenever
the button of the press is pushed in, and stop when the pressure is
released. In this way of course the trumpet will act as a "call"
instead of a bell, and as the double wire can be easily hidden under
the carpet and in dark corners, and painted to match whatever wood-work
it crosses, you can arrange it from an up-stairs room to a down-stairs
one or _vice versa_ with very little trouble. I give an illustration
of the method of connecting the battery and trumpet with one switch or
"press," to show how to arrange the series. (See Fig. 3.)
[Illustration: FIG. 3.--METHOD OF CONNECTING TRUMPET TO BATTERY AND ONE
PRESS.]
The trumpet made in the very simple way I have described will not
produce a very loud noise, but quite loud enough, if properly put
together, to attract a person's attention who was in the room when it
went off. The sound can be rendered louder by fixing a cardboard funnel
or "cornucopia" to the front of the tooth-powder box to make a kind of
horn.