I have a box of used meters bought at a rally which had no markings on their bodies. In order to make use of these I had to find out a little about their properties...

Finding the internal resistance:

• Remove VR2 from the circuit
• Adjust VR1 for full-scale deflection (FSD)
• Re-insert VR2 and adjust this for half-scale deflection
• Remove VR2 and measure it's resistance
• This will now be equivilant to the meter's internal resistance

• Adjust VR1 for FSD
• Measure the resistance of VR1
• Add this to meter's resistance
• Current used = Voltage divided by Resistance (V/R=I)

For example...

• Internal resistance = 1,000 Ohm's
• Required current = 500mA
• Actual current = 1mA
• Thus: 0.5A divided by 0.001A = 500
• Shunt resistance = 1,000 Ohm's (meter resistance) divided by 499 (500 - 1)
• Equals 2.004 Ohm's