Home Made Ammeter and Working of Ammeter-Ammeter Assembly:
Introduction
An ammeter is a device that detects an electric
current. With some readily available materials, you can build your own
ammeter and use it to measure current produced by batteries including
homemade batteries and generators.
Ammeter
Material
- Magnet wire, 10 meters or more of 22 gauge or higher, (available
from Radio Shack). (The higher the gauge, the finer the wire, and the
harder it will be to wind.)
- two small disk magnets (1 cm diameter available from Radio Shack)
- thread or fishing line
- a cardboard tube (e.g. from a toilet paper roll.)
- a Base, a piece of corrugated cardboard about 10 cm x 10 cm
- (For a more rugged meter use wood.)
- Hot melt glue (or staples)
- Aluminum foil, two 5 cm squares
- Tacks or pushpins
- Two alligator clip leads (available at Radio Shack)
- Sandpaper
- AA battery (almost any battery will work)
Assembly
Cut a 4 cm (1.5 inch) length from the tube.
Wind a coil of at least 100 turns of magnet wire around the section
of cardboard tube, leave at least 10 cm of wire free at each end of the
coil. Use masking tape or hot melt glue to hold the coil in place on
the tube.
Hot melt glue, or staple, the paper tube to the base.
Sandpaper the ends of the magnetic wire to remove the insulating
layer of enamel. The ends of the wire should appear as shiny copper.
Take one free end of the wire from the coil make it into a loop and
twist the loop several times. Fold the
aluminum foil around the looped bare ends of the wire until it makes a
rectangle, several layers thick and about 1 cm x 2.5 cm. Use a pushpin
or staple the
aluminum foil pad to the cardboard. Do the same with the other free
end of the wire. Mark a + sign near one end of the wire, it does not
matter which end.
Hang the magnet from a string in the center of the coil: ,make a
string sandwich, hold the string between the two disk magnets. Make a
slit in the center of the top of the tube, run the slit up to the edge
of the coil. Make another slit on the other side of the coil. Hang the
magnet by slipping its thread through the first slit then slip the
thread through the second one. The friction of the thread going through
two slits will hold the magnet in place. Adjust the thread so that the
magnet hangs in the middle of the coil.
To do and notice
The hanging magnet aligns itself with the earth's magnetic field,
just like a compass needle. (see the activity titled, "Where's North?")
To use the ammeter, turn the coil so that the coil is lined up north
and south. When you look in the end of the tube you should see the edge
of the magnet.
End view of the ammeter with the magnets aligned with the Earth's magnetic field.
Attach the battery to the leads. Attach the + side of the battery to
the + lead of the meter. The hanging magnet should turn so that one face
is toward the
aluminum pads. Attach a piece of tape to that face and print + on the
tape.
Reverse the connections to the battery the magnet should rotate the opposite direction.
Your ammeter is very sensitive. The magnets in ours deflect 45
degrees when 10 milliamps flow through them. To measure the sensitivity
of your meter connect a calibrated ammeter in series with a battery and a
variable resistor. Change the resistance until the ammeter magnet turns
45 degrees read the current on the calibrated ammeter.
You can use your ammeter to measure the current produced by your own lemon batteries or home-made generators.
What's Going On?
The electric current running through the coil of wire produces a
magnetic field. The coil has a north pole end and a south pole end. The
magnetic field of the electromagnet pushes on the the hanging magnet,
making it twist. The hanging magnet is held in its no-current position
by the earth’s magnetic field. The stronger the current in your coil,
the stronger the magnetic field produced by the coil and the more the
magnet twists.
When the magnetic field of the electromagnet equals the magnetic
field of the earth the magnets will hang at a 45 degree angle from a
line down the axis of the tube.
Going Further
Measure the current through a lamp from a AA battery, try a 2 volt Christmas tree light from a string of 50 lights.
Measure the current through a motor made by a AA battery.
Measure the current from a lemon battery.
Measure the current from a solar cell.
Alternative construction
Build the same as above but use PVC tubes 1 inch in diameter instead of cardboard tubes.
or
with a compass
Wrap a coil of 100 turns of wire around the
compass. Wind the coil so that it passes over the north and south end of
the compass, and yet still allows you to see the compass needle.
Making the ammeter quantitative
Replace the earth with the largest magnet you can find.
A large magnet can be used to calibrate the ammeter.
Use a calibrated ammeter in series with your ammeter and move the
large magnet until the central disk magnets make a 45 degree angle with
the axis of the tube. Then make a mark on the base to indicate the
position of the large magnet.
