Galvanometers, Ammeters, Voltmeters. How to Make a Galvanometer.
If several turns of wire are wrapped about a compass, and a current of
electricity sent through the coil, the compass needle will be deflected.
Such an instrument is called a *galvanometer* and may be used for
detecting very feeble currents.
Figure 67 shows a very simple form of galvanometer which may be made by
winding about fifty turns of No. 36 B. & S. Gauge single silk covered
wire around an ordinary pocket compass. The compass is set on a block of
wood and the wire wound around both the compass and the block. The
terminals of the wire are connected to two binding posts.
[Illustration: FIG. 67.—A Simple Galvanometer.]
A galvanoscope of this sort may also be used for a polarity indicator to
distinguish between positive and negative poles. Connect an ordinary dry
cell to the galvanometer and the needle will immediately swing to one
side or the other, depending upon which way the current flows. The
carbon of the dry cell is *positive* and the zinc is *negative*. Note
which way the needle swings, whether toward the binding post connected
to the positive pole or the one connected to the negative pole. It will
swing towards the corresponding pole when any other source of current is
connected to it and by knowing whether it points towards positive or
negative it is very easy to determine the polarity.
The Construction of Ammeters and Voltmeters.
The experimenter will find a voltmeter and an ammeter to be very useful
instruments about the workshop and laboratory. A *voltmeter* is an
instrument designed to measure electro-motive force or electrical
pressure. An instrument designed to measure rate of flow of current is
called an *ammeter*.
Ammeters and voltmeters are really, in principle, galvanometers, the
scales of which have been calibrated to read in amperes or volts, as the
case may be.
The little meters described below are very simple but quite sensitive.
[Illustration: FIG. 68.—Details of the Bobbin.]
The wooden bobbin which holds the wire is shown in Figure 68. The exact
dimensions are best understood from the illustration. The wood can be
easily secured from an old cigar box. In laying out the work, scratch
the lines on the wood with the point of a darning needle. Pencil lines
are too thick to permit of accuracy in small work. The bobbin should be
perfectly square and true when finished. Finish by rubbing with fine
sandpaper and then give it a coat of shellac. Two bobbins will be
required, one for the voltmeter and one for the ammeter. Do not use any
nails in putting the bobbins together. Use strong glue only.
The bobbin for the ammeter should be wound full of No. 14 B. & S. Gauge
double cotton covered magnet wire. The voltmeter requires much smaller
wire. No. 37 B. & S. Gauge single silk covered wire will serve the
purpose satisfactorily, but a finer size such as No. 38 or 40, is
better. Such fine sizes will probably, however, prove more difficult for
the experimenter to obtain. Sufficient wire should be used to fill the
bobbin up. The wire should be wound on carefully in smooth even layers.
A small hole should be bored in one of the flanges, through which to
pass the end of the wire when starting the first layer. About six inches
of wire should be left at both ends of the coils to make connection to
the terminals with. The whole winding should be given a coat of shellac
when finished.
[Illustration: FIG. 69.—Details of the Armature, Bearings and Pointer.]
The armature is a piece of soft steel, one and one-eighth inches long,
and one-quarter of an inch thick. A one-sixteenth inch hole is bored,
slightly above the centre of the armature, to receive the shaft. The
centre of gravity is thus thrown below the centre of mass and the
pointer attached to the armature will always return to zero if the
instrument is level. The shaft is a piece of one-sixteenth inch bessemer
steel rod, three-eighths of an inch long. The ends are filed to a sharp
point as shown in the upper pert of Fig. 69. A small hole is bored in
the top of the armature almost directly over the shaft as shown in Fig,
69 to receive the lower end of the pointer, which is a piece of No. 16
aluminum wire, three and one-quarter inches long. The other end of the
pointer should be flattened slightly by tapping with a hammer and then
filed into a shape resembling a spear head.
[Illustration: FIG. 70.—A complete Voltmeter having the Scale at the
top.]
After all the holes have been bored and before the shaft and the pointer
are in place, the armature must be tempered so that it will retain its
magnetism. In order to temper the armature, heat it to a bright red and
drop it immediately into a basin of strong salt water. It may then be
magnetized by rubbing one end against the pole of a strong magnet.
The bearings are formed by two 6-32 brass screws, one-half an inch long
and having a small hole in the end to accommodate the end of the shaft.
These screws pass through the upper flange of the bobbin from the
opposite sides. The holes in the flanges should be slightly smaller than
the outside of the screw so that the latter will fit snugly and "take
hold" as if the wood were threaded.
Figures 70 and 71 show two different methods of assembling and
completing the instruments. In one, the bobbin is mounted on the base
and the scale is at the top. In the other, this relation is just turned
abound and the bobbin is at the top and the scale at the bottom. In the
latter case the pointer must be attached to the bottom of the armature
instead of the top.
[Illustration: FIG. 71—An Ammeter so constructed that the Scale is at
the bottom.]
Figure 73 shows the shapes and dimensions of the wooden parts which
compose the case.
A glass front slides in two shallow grooves cut in the wooden sides,
one-eighth of an inch from the front. Glue and brass screws should be
used in putting the case together. Do not use iron or iron screws.
[Illustration: FIG. 72.—Showing how the Armature, Shaft and Pointer are
assembled for a Meter having the Scale at the bottom.]
The two binding posts connected to the terminals of the wire on the
bobbin should be mounted on the base. A small, round-headed brass screw,
long enough to pass all the way through the base will serve to level the
instrument and bring the pointer exactly at zero. If a little brass
strip is placed in the slot in the screw head and soldered there so as
to form what is known as a "winged screw", the adjustment may be made
with the fingers and without the aid of a screw driver.
The scale is formed on a piece of stiff white cardboard directly under
the pointer. The scale is supported by gluing it to two small wooden
blocks. The various values are marked on the scale with a pen and ink.
The glass front should therefore not be put in place until the
instrument has been calibrated.
To properly calibrate the meters, they must be compared with a suitable
standard.
The zero value on the meters is normally in the centre of the scale.
When a current is passed through the bobbins, the armature tends to
swing around at right angles. But since the armature is pivoted above
its centre of mass, the centre of gravity is displaced when it swings
and exerts a pull in opposition to that of the bobbin. The amount of
swing will be greater as the current is correspondingly stronger. The
pointer will swing either to the right or to the left, depending upon
the direction in which the current passes through the coil of wire on
the bobbin.
[Illustration: FIG. 73.—Details of the Wooden Parts which form the
Case.]
The zero point on the two instruments shown in the accompanying
illustration is at the extreme left of the scales. The pointers are bent
to the left so that the current may be passed through the meters in only
one direction and the scales will have a greater range of values.
In order to calibrate the ammeter, place it in series with a standard
ammeter having a calibrated scale which is known to be correct. A set of
strong batteries and a rheostat should be included in the circuit. The
arrangement is illustrated in Figure 74. The rheostat is adjusted so
that various current readings are obtained. The positions of the pointer
on the standard meter are carefully noted and corresponding graduations
made on the scale of the other meter for each value.
[Illustration: FIG. 74.—Showing how the Apparatus is arranged and
connected for calibrating the Ammeter.]
[Illustration: FIG. 75.—Showing how the Apparatus is arranged and
connected for calibrating the Voltmeter.]
In order to calibrate the voltmeter it must be compared to a standard
voltmeter. The voltmeters must be placed in parallel or shunt with each
other as shown in Figure 75.
A switch is connected to the battery so that the voltage of a varying
number of cells may be passed through the meters. In order to secure a
close adjustment of the voltage, a rheostat is placed across the battery
and switch so that it shunts the cells which are in circuit. By
adjusting both the rheostat and the switch, any voltage within the
maximum range of the battery may be secured.
After the meters have been calibrated and assembled, they are ready for
service and will form a very useful and valuable part of the
experimenter’s laboratory.
When using the meters remember that it is always necessary that the
ammeter shall be in series and the voltmeter in parallel or shunt with
the circuit.