An instrument designed to measure electromotive force (electrical
pressure) is called a _voltmeter_. An instrument designed to measure
volume of current is called an _ammeter_.
There are many forms of reliable meters for measuring current and
voltage, but all are more or less expensive and out of the reach of an
ordinary boy.
Some meters are more carefully made than a watch, and are provided with
fine hair-springs and jeweled bearings, but all depend upon the same
principle for their action, namely, the mutual effects produced between
a magnetic needle and a coil of insulated wire carrying a current of
electricity.
The little meters described in this chapter are simple and inexpensive
but quite sensitive. Unlike a meter making use of a hair-spring, they
will stand considerable rough handling, but of course should not be
subjected to such treatment unnecessarily.
Two types of meters are described. Both operate on exactly the same
principle, but one is more elaborate than the other.
A Simple Voltmeter and Ammeter
A base-board five inches long, two and one-half inches wide and one-half
inch thick is cut out of hard wood. In its center, cut a slot
three-eighths of an inch wide and one and one-half inches long, with the
slot running lengthwise the board. Along each side of the slot glue two
small wooden blocks one and one-half inches long, one-quarter of an inch
thick, and one-half of an inch high.
[Illustration: Fig. 102.—_A_, Base, showing Slot. _B_ and _C_, Sides and
Top of the Bobbin. _D_, Base and Bobbin in Position.]
When they are firmly in position, glue a strip of wood, two and one-half
inches long, three-quarters of an inch wide and one-eighth inch thick to
the top as shown by D in Figure 102.
Using these as a support, wind a horizontal coil composed of 200 feet of
No. 36 B. & S. gauge silk-covered wire.
A needle is next made from a piece of watch-spring. It should be about
one and one-quarter inches long, and one-eighth of an inch wide.
Straighten it out by bending, and then heat the center in a small
alcohol flame until the center is red-hot, taking care to keep the ends
as cool as possible.
The spring is mounted on a small steel shaft made by breaking up an
ordinary sewing-needle. Make the piece one-half of an inch long. It must
have very sharp points at both ends. The ends may be pointed by
grinding.
[Illustration: Fig. 103.—Arrangement of the Needle and Pointer.]
Bore a small hole just large enough to receive the needle through the
center of the spring. Insert the needle in the hole and fasten it in the
center by two small circular pieces of wood which fit tightly on the
needle. A little glue or sealing-wax will serve to help make everything
firm.
The pointer is a piece of broom-straw, about three inches long. Bore a
small hole in the top of one of the wooden clamps and insert the pointer
in the hole, fastening it with a little glue. The pointer should be
perfectly straight, and in a position at right angles to the spring.
Bore a small hole in the bottom of one of the wooden clamps and glue a
small wire nail in the hole. The purpose of the nail is to serve as a
counterweight and keep the pointer in a vertical position.
The spring should be magnetized by winding ten or twelve turns of magnet
wire around one end and connecting it with a battery for a moment.
[Illustration: Fig. 104.—_A_, Bearings. _B_, How the Needle is mounted.]
The needle is mounted in two small pieces of thin sheet-brass, one inch
long and one-half inch wide. Bend each strip at right angles in the
middle, and at one-quarter of an inch from one end make a small dent by
means of a pointed nail and a hammer.
The strips are now slipped down in the center of the slot in the coil
with the dents inside of the coil and exactly opposite one another.
After the exact position is found, they may be fastened into position by
two very small screws.
The sharp-pointed sewing-needle, together with the magnetized spring,
pointer, and counterweight, should slip down into the dents made in the
strips and swing freely there. It may require a little filing and
bending, but the work should be done patiently, because the proper
working of the meter will depend upon having the needle swing freely and
easily in its place.
Fasten an upright board, four inches wide and one-quarter of an inch
thick, to the base-board, back of the bobbin.
Attach a piece of thick cardboard to the upright by means of small
blocks, in such a position that the pointer swings very close to it but
does not touch it.
The meter is now complete, except for marking or calibrating the scale.
The method of accomplishing this will be described farther on.
[Illustration: Fig. 105.—The Completed Meter.]
If the meter is wound with No. 36 B. & S. gauge wire it is a voltmeter
for measuring voltage. If it is wound with No. 16 B. & S. gauge wire it
will constitute an ammeter for measuring amperes.
A Portable Voltmeter and Ammeter
The bobbin upon which the wire is wound is illustrated in Figure 106.
The wood is the Spanish cedar, of which cigar boxes are made. It should
be one-eighth of an inch thick, and can be easily worked with a
pocket-knife. 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 when finished must be perfectly true
and square.
The dimensions are best understood from the illustrations. In putting
the bobbin together, do not use any nails. Use strong glue only.
Two bobbins are required, one for the ammeter and one for the voltmeter.
After completing the bobbins, sandpaper them and coat them with shellac.
[Illustration: Fig. 106.—Details of the Bobbin.]
The bobbin for the ammeter is wound with No. 14 B. & S.
double-cotton-covered magnet wire. The voltmeter requires No. 40 B. & S.
silk-covered wire. In both cases the wire should be wound carefully in
smooth, even layers. A small hole is bored in the flange through which
to pass the end of the wire when starting the first layer. After
finishing the winding, about six inches of wire should be left at both
ends to make connection with the terminals. The whole winding is then
given a coat of shellac. A strip of passe-partout tape, one-half of an
inch wide wound over the wire around the bobbin will not only protect
the wire from injury, but also give the bobbin a very neat appearance.
The armature is a piece of soft steel one inch long, one-eighth of an
inch thick and three-eighths wide. A one-eighth-inch hole is bored
one-sixteenth of an inch above the center for the reception of the
shaft. The center of gravity is thus thrown below the center of the mass
of the armature, and the pointer will always return to zero if the
instrument is level.
The shaft is a piece of one-eighth-inch Bessemer steel rod,
seven-sixteenths of an inch long. The ends are filed to a sharp
knife-edge on the under side, as indicated in the figure.
[Illustration: Fig. 107.—The Bobbin partly cut away so as to show the
Bearing. Details of the Armature and Shaft.]
A one-sixteenth-inch hole is bored in the top of the armature to receive
the lower end of the pointer, which is a piece of No. 16 aluminum wire,
four and one-half inches long.
After the holes have been bored, the armature is tempered so that it
will retain its magnetism. It is heated to a bright red heat and dropped
into a basin of strong salt water. The armature is then magnetized by
rubbing one end against the pole of a strong magnet.
The bearings are formed by two strips of thin sheet-brass,
three-sixteenths of an inch wide, and one and one-quarter inches long,
bent and glued to the sides of the bobbin.
In the illustration, part of the bobbin is represented as cut away. The
center of the bearing is bent out so that the end of the shaft will not
come in contact with the sides of the bobbin. The top of the center is
notched with a file to form a socket for the knife-edges of the shaft.
[Illustration: Fig. 108.—Completed Voltmeter.]
The bobbin is glued to the center of a wooden base, seven inches long,
four inches wide and three-quarters of an inch thick. The terminals of
the coil lead down through two small holes in the base and thence to two
large binding-posts. The wires are inlaid on the under side of the base,
i.e., they pass from the holes to the binding-posts through two grooves.
This precaution avoids the possibility of their becoming short-circuited
or broken.
The case is formed of two sides, a back and top of one-half-inch wood.
It is six inches high, four inches wide, and two inches deep. A glass
front slides in two shallow grooves cut in the wooden sides, one-eighth
of an inch from the front.
The case is held down to the base by four round-headed brass screws,
which pass through the base into the sides. It is then easily removable
in case it ever becomes necessary to repair or adjust the instrument.
The meter and case, as illustrated in Figure 108, are intended for
portable use and are so constructed that they will stand up. A small
brass screw, long enough to pass all the way through the base, serves to
level the instrument. If a little brass strip is placed in the slot in
the screw-head and soldered 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.
Where the instrument is intended for mounting upon a switch-board, it
can be given a much better appearance by fitting with a smaller base,
similar in size and shape to the top. The binding-posts are then mounted
in the center of the sides.
To calibrate the meters properly, they are compared with some standard.
The scale is formed by a piece of white cardboard glued by two small
blocks on the inside of the case. The various values are marked with a
pen and ink. The glass front, therefore, cannot be put in place until
they are located.
The zero value on the meters will normally be in the center of the
scale. When a current is passed through the bobbin, the armature tends
to swing around at right angles to the turns of wire. But since the
armature is pivoted above the center of the mass, when it swings, the
center of gravity is displaced and exerts a pull in opposition to that
of the bobbin, and the amount of swing indicated by the pointer will be
greater as the current is stronger. The pointer will swing either to the
right or the left, depending upon the direction in which the current
passes through the bobbin. The pointer of the instrument illustrated in
Figure 108 is at zero when at the extreme left of the scale. The pointer
is bent to the left, so that the current will be registered when passing
through the meter only in one direction, but the scale will have a
greater range of values. It will also be necessary to cut a small groove
in the base of the instrument in this case so that the armature will
have plenty of room in which to swing.
[Illustration: Fig. 109.—Circuits for Calibrating the Ammeter and
Voltmeter.]
When calibrating the ammeter, it is placed in series with the standard
meter, a set of strong batteries, and a rheostat. The rheostat is
adjusted so that various current readings are obtained. The
corresponding positions of the pointer on the meter being calibrated are
then located for each value.
The voltmeters must be placed in parallel, or shunt with each other, and
in series with several battery cells. A switch is arranged so that the
voltage of a varying number of cells may be passed through the meters.
To secure fractional values of a volt, the rheostat is placed in shunt
with the first cell of the battery. Then, by adjusting both the switch
and the rheostat, any voltage within the maximum range of the battery
may be secured.
This means of regulating voltage is a common one, and of much use in
wireless telegraph circuits, as will be explained later.
When using the meters, it is always necessary that the ammeter shall be
in series and the voltmeter in parallel or in shunt with the circuit.
Galvanoscopes and Galvanometers
In the first part of Chapter V it was explained that several turns of
wire surrounding a compass-needle would cause the needle to move and
show a deflection if a current of electricity were sent through the
coil.
Such an instrument is called a _galvanoscope_ and may be used for
detecting very feeble currents. A galvanoscope becomes a _galvanometer_
by providing it with a scale so that the deflection may be measured.
A galvanometer is really, in principle, an ammeter the scale of which
has not been calibrated to read in amperes.
[Illustration: Fig. 110.—Simple Compass Galvanoscope.]
A very simple galvanoscope may be made by winding fifty turns of No. 36
B. & S. gauge single-silk-covered wire around an ordinary pocket
compass. The compass may be set in a block of wood, and the wood
provided with binding-posts so that connections are easily made.
Another variety of the same instrument is shown in Figure 111.
[Illustration: Fig. 111.—Galvanoscope.]
Wind about twenty-five turns of No. 30 B. & S. gauge cotton-covered
wire around the lower end of a glass tumbler. Leave about six inches of
each end free for terminals, and then, after slipping the coil from the
glass, tie the wire with thread in several places so that it will not
unwind. Press two sides of the coil together so as to flatten it, and
then attach it to a block of wood with some hot sealing-wax.
Make a little wooden bridge as shown in Figure 111, and mount a
compass-needle on it in the center. The compass-needle may be made out
of a piece of spring-steel in the manner already described in Chapter I.
Mount two binding-posts to the corners of the block, and connect the
ends of the wire coil to them. Turn the block so that the needle points
North and South and parallel to the coil of wire.
If a battery is connected to the binding-posts, the needle will fly
around to a position at right angles to that which it first occupied.
An astatic galvanoscope is one having two needles with their poles in
opposite directions. The word "astatic" means having no directive
magnetic tendency. If the needles of an astatic pair are separated and
pivoted separately, they will each point to North and South in the
ordinary manner. But when connected together with the poles arranged in
opposite directions they neutralize each other.
An astatic needle requires but very little current in order to turn it
either one way or the other, and for this reason an astatic galvanoscope
is usually very sensitive.
A simple instrument of this sort may be made by winding about fifty
turns of No. 30-36 B. & S. gauge single-silk or cotton-insulated wire
into a coil around a glass tumbler. After removing the coil from the
glass, shape it into the form of an ellipse and fasten it to a small
base-board.
Separate the strands of wire at the top of the coil so that they are
divided into two groups.
[Illustration: Fig. 112.—Astatic Galvanoscope.]
Make a bridge or standard in the shape of an inverted U out of thin
wooden strips and fasten it to the block.
The needles are ordinary sewing-needles which have been magnetized and
shoved through a small carrier-bar, made from a strip of cardboard, with
their poles opposite one another, as shown in the illustration.
[Illustration: Fig. 113.—Astatic Needles.]
They may be held in place in the cardboard strip by a small drop of
sealing-wax.
A small hole is punched in the top of the carrier, through which to pass
the end of a thread. The upper end of the thread passes through a hole
in the bridge and is tied to a small screw-eye in the center of the
upper side of the bridge.
The carrier-bar is passed through the space where the coil is split at
the top. The lower needle should hang in the center of the coil. The
upper needle should be above and outside the coil.
The terminals of the coil are connected to two binding-posts mounted on
the base-block.
Owing to the fact that this galvanoscope is fitted with an astatic
needle, the instrument does not have to be turned so that the coil may
face North and South. The slightest current of electricity passing into
the coil will instantly affect the needles.
An astatic galvanometer for the detection of exceedingly weak currents
and for use in connection with a "Wheatstone bridge" for measuring
resistance, as described farther on, will form a valuable addition to
the laboratory of the boy electrician.
Make two small bobbins similar to those already described in connection
with the volt and ammeter, but twice as long, as shown in Figure 114.
Wind each of the bobbins in the same direction with No. 36 silk-covered
or cotton-covered wire, leaving about six inches free at the ends for
connection to the binding-posts.
Fasten each of the bobbins to the base-board with glue. Do not nail or
screw them in position, because the presence of nails or screws may
impair the sensitiveness of the instrument. In mounting the bobbins,
leave about one-sixteenth of an inch of space between the inside
flanges, through which the needle may pass.
Connect the coils wound on the bobbins so that the end of the outside
layer of the first coil is connected to the inside layer of the other
coil. This arrangement is so that the current will travel through the
windings in the same continuous direction, exactly the same as though
the bobbin were one continuous spool.
[Illustration: Fig. 114.—Bobbin for Astatic Galvanometer.]
Magnetize two small sewing-needles and mount them in a paper stirrup
made from good, strong paper, as shown in Figure 114. Take care that the
poles are reversed so that the north pole of one magnet will be on the
same side of the stirrup as the south pole of the other. They may be
fastened securely by a drop of shellac or melted sealing-wax.
Cut out a cardboard disk and divide it into degrees as in Figure 115.
Glue the disk to the top of the bobbins. A small slot should be cut in
the disk so that it will pass the lower needle.
A wooden post should be glued to the back of the base. To the top of
this post is fastened an arm from which are suspended the magnetic
needles.
A fine fiber for suspending the needle may be secured by unraveling a
piece of embroidery silk.
[Illustration: Fig. 115.—Completed Astatic Galvanometer.]
The upper end of the fiber is tied to a small hook in the end of the
arm. The wire hook may be twisted so that the needles may be brought to
zero on the scale. Zero should lie on a line parallel to the two coils.
The fiber used for suspending the needles should be as fine as possible.
The finer the fiber is, the more sensitive will the instrument be.
The lower needle should swing inside of the two coils, and the upper
needle above the disk.
How to Make a Wheatstone Bridge
The amateur experimenter will find many occasions when it is desirable
to know the resistance of some of his electrical apparatus. Telephone
receivers, telegraph relays, etc., are all graded according to their
resistance in ohms. The measurement of resistance in any electrical
instrument or circuit is usually accomplished by comparing its
resistance with that of some known circuit, such as a coil of wire which
has been previously tested.
The simplest method of measuring resistance is by means of a device
known as the Wheatstone bridge. This instrument is very simple but at
the same time is remarkably sensitive if properly made. A Wheatstone
bridge is shown in Figure 116.
The base is a piece of well-seasoned hard wood, thirty inches long, six
inches wide, and three-quarters of an inch thick.
Secure a long strip of No. 18 B. & S. gauge sheet-copper, one inch wide,
and cut it into three pieces, making two of the pieces three inches
long, and the other piece twenty-three and one-half inches long.
Mount the copper strips on the base, as shown, being very careful to
make the distance between the inside edges of the end-pieces just
twenty-five inches. The strips should be fastened to the base with small
round-headed brass screws. Mount two binding-posts on each of the short
strips in the positions shown in the illustration, and three on the long
strip. These binding-posts should pass through the base and make firm
contact with the strips.
[Illustration: Fig. 116.—Wheatstone Bridge.]
Then make a paper scale twenty-five inches long, and divide it into one
hundred equal divisions one-quarter of an inch long. Mark every fifth
division with a slightly longer line, and every tenth division with a
double-length line.
Start at one end and number every ten divisions, then start at the other
end and number them back, so that the scale reads 0, 10, 20, 30, 40, 50,
60, 70, 80, 90, 100, from right to left at the top and 0, 10, 20, 30,
40, 50, 60, 70, 80, 90, 100, from left to right at the bottom.
Solder a piece of No. 30 B. & S. gauge German-silver wire to one of the
short copper strips opposite the end of the scale, and then stretch it
tightly across the scale and solder it to the strip at the other end.
Make a knife-contact by flattening a piece of heavy copper wire as shown
in Figure 117. Solder a piece of flexible wire, such as "lamp cord," at
the other end. It is well to fit the contact with a small wooden handle,
made by boring out a piece of dowel.
The instrument is now practically complete.
[Illustration: Fig. 117.—Knife-Contact.]
In order to use the Wheatstone bridge, it is necessary to have a set of
resistances of known value. The resistance of any unknown circuit or
piece of apparatus is found by comparing it with one of the known coils.
It is just like going to a store and buying a pound of sugar. The grocer
weighs out the sugar by balancing it on the scales with an iron weight
of known value, and taking it for granted that the weight is correct, we
would say that we have one, five, or ten pounds of sugar, as the case
may be.
The Wheatstone bridge might be called a pair of "electrical scales" for
weighing resistance by comparing an unknown coil with one which we know
has a certain value.
The next step is to make up some standard resistance coils. Secure some
No. 32 B. & S. gauge single-cotton-covered wire from an electrical
dealer and cut into the following lengths, laying it straight on the
floor but using care not to pull or stretch it.
1/2 ohm coil—3 feet 1/2 inch
1 ohm coil—6 feet 1 1/4 inches
2 ohm coil—12 feet 2 1/2 inches
5 ohm coil—30 feet 6 1/4 inches
10 ohm coil—61 feet
20 ohm coil—122 feet
30 ohm coil—183 feet
50 ohm coil—305 feet
These lengths of wire are then wrapped on the spools in the following
manner.
[Illustration: Fig. 118.—Resistance-Coil. _A_ shows how the Wire is
doubled and wound on the Spool. _B_ is the completed Coil.]
This method of winding is known as the non-inductive method, because the
windings do not generate a magnetic field, which might affect the
galvanometer needle used in connection with the Wheatstone bridge as
described later on.
Each length of wire should be doubled exactly in the middle, then
wrapped on the spools like a single wire, the two ends being left free
for soldering to the terminals as shown in Figure 118, B.
The spools may be the ordinary reels upon which cotton and sewing-silk
are wrapped.
The terminals of the spools are pieces of stout copper wire, No. 12 or
No. 14 B. & S. gauge. Two pieces of wire about three inches long are
driven into holes bored in the ends of each spool. A small drop of
solder is used permanently to secure the ends of the coil to each of the
heavy wire terminals.
The spools are then dipped into a pan of molten paraffin and boiled
until the air bubbles cease to rise.
The spools should be marked 1, 2, 10, 20, 30, and 50, according to the
amount of wire each one contains as indicated in the table above.
How to Use a Wheatstone Bridge for Measuring Resistance
The instrument is connected as in Figure 116.
The unknown resistance or device to be measured is connected across the
gap at _B_. One of the standard known coils is connected across the gap
at _A_. A sensitive galvanometer or a telephone receiver and two cells
of battery are also connected as shown.
If a telephone receiver is used, place it to the ear. If a galvanometer
is used instead, watch the needle carefully. Then move the sharp edge of
the knife-contact over the scale along the German-silver "slide wire"
until a point is reached when there is no deflection of the needle or no
sound in the telephone receiver.
If this point lies very far on one side or the other of the center
division on the scale, substitute the next higher or lower known
resistance spool until the point falls as near as possible to the center
of the scale.
When this point is found, note the reading on the scale carefully. Now
comes the hardest part. Almost all my readers have no doubt progressed
far enough in arithmetic to be able to carry on the following simple
calculation in proportion which must be made in order to find out the
resistance of the unknown coil.
The unknown resistance, connected to _B_, bears the same ratio to the
known coil, at _A_, that the number of divisions between the
knife-contact and the right-hand end of the scale (lower row of figures)
bears to the number of divisions between the knife-edge and the
left-hand end of the scale (upper row of figures).
We will suppose that a 5-ohm coil was used at _A_ in a test, and the
needle of the galvanometer stopped swinging when the knife-contact
rested on the 60th division from the left-hand end, or on the 40th from
the right. Then, in order to find the value of the unknown resistance at
_B_, it is simply necessary to multiply the standard resistance at _A_
by the number of left-hand divisions and divide the product by the
number of right-hand divisions. The answer will be the resistance of _B_
in ohms.
The calculation in this case would be as follows:
5 X 40 = 200
200/60 = 3.33 ohms
3.33 ohms is the resistance of _B_.
This explanation may seem very long and complex, but if you will study
it carefully you will find it to be very simple. When once you master
it, you will be enabled to make many measurements of resistance which
will add greatly to the interest and value of your experiments.