How to Convert Ammeter to Voltmeter
- 1). Insert a resistor in the internal circuit. This reduces the amount of current to flow through the voltmeter itself, so that it doesn't significantly affect the circuit being measured.
- 2). Remove any parallel shunt resistor the ammeter may contain. These are included in ammeters to maximize current flow through the ammeter. In other words, most of the current flows through the parallel resistor of little resistance, and the little that flows through the galvanometer registers current that it translates into current through the shunt resistor, which has negligible resistance.
- 3). Replace the permanent magnet or the spring on the electromagnet to get the range of volts desired. Use the following equality to help: NiAB = κφ. Here, the left-hand side is the magnetic torque on the electromagnet holding the gauge needle. The right-hand side is the opposing spring torque at deflection φ. A is the area of the coil of the electromagnet, i is the current through the N loops of the electromagnet's wire, B is the permanent magnetic field's stength, and κ is the spring force constant. So if the range of the needle is, say, 30 degrees, then φ = 30 degrees for the current, i, that corresponds to the maximum voltage drop, V, across the voltmeter that you want the meter to be able to register. You find i from V by the definition of resistance R = V/i, and vary the resistance R of the voltmeter to get the desired voltage range.
- 4). Replace the current units on the gauge with voltages, per your calibration in step 5.
