Shunts and Multipliers: One Movement, Every Range

Connect the panel meter in series with a variable resistor (a potentiometer) and a 1.5 V cell, and adjust for exactly full-scale deflection. Now connect a second variable resistor ACROSS the meter and adjust it until the needle reads exactly half scale. Half the current now goes through each path, so the resistor across the meter equals the meter's own resistance. Disconnect it and measure it with the multimeter: that is R_m. Put it, and the full-scale current from the meter's dial, into the notebook.

For a 10 V voltmeter, choose resistors from the kit that add up to the notebook's multiplier value, put them in series with the meter, and check the reading against the multimeter on a battery or the bench supply. For a 1 A ammeter, cut the notebook's length of copper wire (about 2.3 m of 0.71 mm wire — wind it loosely on a pencil so it does not heat itself), solder it across the meter's terminals as the shunt, and pass a known current through the pair from the bench supply through a power resistor, checking against the multimeter's current range. Trim the wire until the readings agree. Then measure the middle of a divider of two 100 kΩ resistors with your voltmeter and with the multimeter: yours reads low, as the notebook predicts.

A practice rung: shunts and multipliers came with the moving-coil meter of the previous rung and turned one movement into a multi-range instrument. The resistance per volt of a meter became its quality mark. **Honest limits.** Copper shunts drift with temperature (manganin does not). Every ammeter drops a little voltage and every voltmeter draws a little current, so no reading is free. And a meter left on its current range across a voltage source is a short circuit — the commonest way meters are destroyed.