Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds

Current balance

The current balance should consist of:

  • base assembly with coil
  • 1 cm Alcomax magnet
  • drinking straw
  • sewing needle
  • rider of tinned copper wire
  • wooden strip, 15 cm x 2 cm
  • wooden block, 4 cm x 4 cm x 2 cm

Fasten the 1 cm magnet near to the end of the drinking straw with about 3 cm of Sellotape. Perhaps the easiest way is to cut the Sellotape, attach it to the magnet and fix the drinking straw as shown below. The current balance

a Balance the straw on the needle to find the centre of gravity of the straw with the magnet attached. Stick the needle through the straw a little farther away from the magnet (not more than 1 mm from the centre of gravity of the combined straw and magnet). If students are making balances themselves do not give these details as instructions. Instead, the teacher should give extra straws and encouragement.
b Then, with the magnet near the centre of the coil, the needle is about 0.5 cm from the end of the metal channel. The straw should be horizontal when the magnet is vertical.
c Make a rider by wrapping 2.5 cm of 20 or 28 SWG copper wire round a pencil and slipping it over the straw. The rider should balance the straw horizontally when it is not more than 5 cm from the needle. If it has to be nearer the free end of the straw, the range of the current balance will be reduced (though not the sensitivity).
The straw should rest horizontally and a reference mark should be made on the wooden strip, attached to a wooden block provided, positioned at the end of the straw.
For the more clumsy students it may be better to use up to 5 cm of 26 or 28 SWG wire for the riders. This will make the balance less sensitive but the position of the pin will be less critical. Provide a short length of the same wire to use as a 'lifter' when adjusting the position of the rider (best done by steadying the straw with one hand and handling the wire and rider with the other).
d Check that the coil does not obstruct the free motion of the straw.
 The current balance