| This gives practice in using
Ohm's Law and Kirchoff's Laws.
The coil of a moving coil meter typically has a resistance of 100 ohms. It needs a current of 1 mA to deflect the pointer fully across the scale (full scale deflection, f.s.d.). From Ohm's Law, the voltage to give f.s.d. is I x R = 0.001 x 100 = 0.1 volts. |
| To enable the meter to indicate
10 mA at f.s.d. Kirchoff's Current Law tells us that 9 mA must be diverted
by means of a SHUNT resistor.
We know the voltage across the shunt (0.1 volt) and the current through the shunt (9 mA). From Ohm's Law we can calculate the resistance of the shunt. 0.1volts/0.9 mA = 0.111 ohms. This low resistance can be wound from wire. You will need wire tables to do this. With several shunts and a muli-way switch you can build a multi-range ammeter. |
Copyright Graham Knott 1999