How to Make a Pole Changing Switch or Current Reverses How to Reverse a
Small Motor.
The switch shown in Figure 76 is a very simple device which will be
found very handy for many purposes. It consists of two brass levers
mounted alongside of each other and connected together with an
insulating strip or yoke bearing a handle so that they can both be moved
together. The levers are pivoted at their back ends and provided with
binding posts marked B. P. and B. P. in the illustration. The front ends
of the levers swing over three switch contact points properly spaced so
that the levers touch the adjoining contacts at the same time. The
contacts are marked 1, 2, and 3.
[Illustration: FIG. 76.—A Pole changing Switch for reversing Small
Motors or the direction of an Electric Current.]
We will suppose that the experimenter has some device operated by a
battery and desires to arrange the apparatus so that he can quickly
reverse the terminals of the battery so as to send the current through
the device in either direction quickly and at will. The apparatus to be
operated should be connected to the two binding posts mounted on the
switch levers and marked B. P. and B. P. Connect the negative pole of
the battery to the contact marked 2 (the center one), and the positive
pole to both 1 and 3 (the outside ones). When the switch is thrown to
the right so that the levers rest on the contacts 2 and 3, the right
hand lever will be positive and the left hand one negative. When the
switch is thrown to the left so that the levers rest on 1 and 2, the
left hand lever will be positive and the right hand one negative. The
current has been reversed.
[Illustration: FIG. 77.—Top view of a small Battery Rheostat]
A switch such as this makes it possible to quickly reverse the running
direction of small motors. In order to accomplish this, connect the two
brushes on the motor to the two switch levers. Connect the contracts 1
and 3 (the two outside ones) together and lead the wire to one terminal
of the battery used for operating the motor. In case the motor is
operated by a generator or from the 110 volt current, connect the wire
to one terminal of the power source. The middle contact on the switch
(marked 2) should be connected to one end of the field winding on the
motor. The other end of the field winding should be connected to the
remaining battery terminal or power source.
Moving the switch back and forth will send the current through the
commutator in opposite directions and cause the motor to run in either
direction as desired.
How to Make a Small Battery Rheostat for Regulating the Speed of Small
Motors, Etc.
*A Rheostat* is a variable resistance for regulating the amount of
current flowing in a circuit. It is a very useful device about the
experimental laboratory. It may be used, for regulating the speed of
small motors, the amount of current flowing into a storage battery when
recharging, the brilliancy of a lamp, etc.
[Illustration: FIG. 78.—Details of the Rheostat Base. The lower part of
the illustration is a cross section.]
Figure 77 shows a form of small rheostat which may be easily built by
the experimenter. It consists of a coil of German silver resistance wire
wound around a fibre form and set in a groove in a wooden base. A brass
lever slides over the edge of the resistance coil so that the portion
included in the circuit may be increased or decreased easily at will.
The dimensions shown in the several drawings which follow are for a
rheostat having a resistance of approximately ten ohms and suitable for
general experimental work. It is possible to use the same plans and by
doubling the dimensions, make a rheostat which is much larger and may be
used to carry heavier currents or which possesses more resistance.
[Illustration: FIG. 79.—Looking at the Base from the bottom showing the
grooves in which the Wires are laid.]
The details of the wooden base are given in Figures 78 and 79. The
outside diameter of the base is four inches. It is five-eighths of an
inch thick. The lower 7 part of Figure 78 shows a cross section of the
base. The groove is three and one-eighth inches in diameter inside,
five-sixteenths of an inch thick and five thirty-seconds of an inch
wide. The hole in the centre of the base is to accommodate the shaft of
the switch lever, Two holes, seven-eighths of an inch apart are drilled
at the back of the base to accommodate the binding posts. The binding
posts’ holes and also the hole in the centre of the base for the shaft
are countersunk at the bottom as shown in Figure 79.
The fibre strip which supports the German silver resistance wire is
shown in Figure 80. It is eight inches long, three-eighths of an inch
wide and one-eighth of an inch thick.
If the experimenter has access to a lathe it is possible to wind the
wire on the fibre strip under tension with the aid of the screw feed so
that the wire will go on very tightly and with a small air space between
the turns. When a lathe is not available it will be necessary to groove
the top and bottom of the fibre strip with the aid of a three-cornered
file. The grooves should be evenly spaced and as close together as
possible. They serve to prevent the wire from slipping and the adjacent
turns becoming short circuited.
The wire used to wind the strip should be No. 24 B. & S. Gauge German
Silver wire and should be bare. Wind the wire tightly into the grooves
in the strip and fasten both ends by looping through a small hole bored
in the ends of the fibre for that purpose.
The fibre strip can be bent so that it will fill in the groove in the
wooden base by heating it in the flame of a bunsen burner. It should
not, of course, be heated too hot or the fibre will burn. Warm it
thoroughly and bend into a circle of the same diameter as the groove in
the base. Then force the strip into the groove with the two ends
opposite the two binding post holes. The strip should fit into the
groove tightly so that there is no danger of it shifting or slipping
out.
[Illustration: FIG. 80.—The German-silver Resistance Wire is wound
around a Fibre Strip.]
The details of the switch lever, knob and binding posts, are shown in
Figure 81. The lever is made of sheet brass cut into the shape shown and
is one and three-quarter inches long. The knob is similar to those used
on typewriters for turning the paper roller. It is fitted with a
threaded stem moulded into the head. These knobs may be obtained at
almost any supply house carrying goods for experimenters. The lever is
slipped over the stem and clamped tightly against the knob by a nut on
the under side. The end of the stem or shaft fits into the hole in the
centre of the wooden base. Slip a washer over the end and then thread on
a brass washer. Tighten the nut up until the end of the lever makes firm
contact with the resistance wire but is not so tight that it binds and
cannot be easily moved by turning the knob. When the right position for
the nut is found, solder it to the stem so that it cannot change its
adjustment.
[Illustration: FIG. 81.—The Lever, Knob, Binding Posts, etc.]
The completed rheostat is shown in Figure 82. The right hand terminal of
the German Silver wire is connected to the right hand binding post. The
other binding post is connected to the washer around the lever shaft on
the under side of the base. The wires should be laid in grooves cut in
the bottom of the base for that purpose.
Drive a brass headed upholsterer’s nail into the base near the left hand
terminal of the resistance so that when the lever is swung around in
that direction as far as it will go, the tack will raise the lever up
off the resistance wire and break the circuit completely. Two small
nails should be driven into the base in such a position that they will
limit the swing of the lever and prevent it from moving so far that it
passes over the ends of the resistance unit.
The rheostat may be connected in a circuit by attaching two wires to the
binding posts. It should be placed in series with any device which it is
desired to regulate. The carrying capacity is limited to two amperes
owing to the fact that a heavier current than this will cause the wire
to overheat. Turning the knob so that the lever moves towards the
upholsterer’s nail, which raises it up off the wire, increases the
amount of resistance in the circuit and cuts down the current, until the
lever slides up on the nail when the circuit is broken entirely. Moving
the knob in the other direction decreases the resistance and increases
the current until the lever has moved as far as it may, when the current
will be at a maximum.
[Illustration: FIG. 82.—The completed Rheostat.]