Moving coil meter
A moving coil meter makes use of the catapult field.
Apparatus and materials
For each student group
Copper wire, PVC-covered, 150 cm with bare ends
Rheostat (10-15 ohms, rated at 5A or more)
Health & Safety and Technical notes
Kits are commercially available for making model electric motors; this experiment is a variation on them.
The armature is made from a wooden or plastic block with an aluminium tube through the clearance hole drilled through the wood.
The base is also of wood or plastic, with holes positioned to take the rivets which hold the wires.
A good length of wire must be used for each spiral. If they are too tight (because the overall length is too short), the meter will be insensitive.
Care should be taken to ensure that the turns of the spiral do not foul each other, or the supports, or the magnets. The leads must also be fixed rigidly to the rivets, so that moving the connecting leads does not move the spirals.
a Wind a coil of ten turns of PVC-covered copper wire on the armature block, with a couple of tight turns round the tube at the end in order to fix the ends. Plenty of wire should be left at each end.
b Coil each end into a loose spiral of four to five turns.
c Pass the knitting needle through the aluminium tube in the armature. Support it above the wooden base using the two split pins.
Check that the knitting needle is firmly held, and that the armature can turn freely. (The split pins can be rotated so that the knitting needle jams in the eye of the split pin; this encourages rigidity.)
d Put rivets in the holes in the wooden base. Fix the leads of the spirals, passing them twice under each rivet head, as shown above. Check that, when you rotate the armature, it springs back approximately to the horizontal position.
e Insert a drinking straw in the hole in the armature block so that it sticks up to act as an indicating needle.
f Place the slab magnets on the yoke, ensuring that opposite poles are facing each other. This provides the magnetic field.
g Slip the yoke into place under the wooden base.
h Connect the leads to the DC terminals of the power supply.
i Switch on; the needle will go hard over.
j Add a rheostat (10-15 ohms, rated at 4 to 5 A) in series with the moving coil meter, so that the deflection can be varied by varying the current. Check that reversing the current reverses the deflection.
1 This goes much better if you have already built a model and then challenge students to produce their own. It can be a bit fiddly, especially for large fingers, but once one student has got it working then others soon follow.
Students will be amazed that, with care, they can build an ammeter which responds to a varying current in a circuit consisting of a cell, lamp, potentiometer and the model ammeter.
2 Once students have their own working model, they can be asked to describe the electromagnetic forces which act on the armature, producing a force couple which causes the armature to turn. This couple is counterbalanced by the couple produced by the spring, so that the armature settles into a position with the two couples balanced.
3 The meter could be tried on AC. It will be found to buzz furiously, but will give no deflection.
4 After students have made their own moving coil instruments, they should look at and examine any available commercially made galvanometers or moving coil ammeters, observing the moving coils within them.
5 This experiment can act as an introduction to electric motors. Ask students, 'Can the ammeter's motion be made to continue?' The motion of the armature is certainly rotary, but it ends when the two forces, the restoring forces of the springs, and the deflecting forces of the magnetic fields, balance. 'If the springs were removed, what then?' The armature would move until it could no longer continue, because the couple produced by the magnetic forces no longer rotated it. If it did rotate, because of its momentum, then the leads would become tangled. (To solve this problem a commutator would need to be constructed.)
This experiment was safety-checked in July 2007