Ammeters and Voltmeters

Friday 7-12-96

The relevant section in the textbook is 17.4

Main concepts:

• An ammeter is a device for measuring current, while a voltmeter is a device for measuring voltage (i.e., potential difference)

• An ammeter is placed in series in a circuit, so it must have a low resistance to prevent it from affecting the current in the circuit to a significant extent.

• A voltmeter is placed in parallel in a circuit, so it must have a high resistance to prevent it from significantly affecting the circuit.

  +++ Anything below here is just for interest +++

• Both ammeters and voltmeters rely on a device called a galvanometer, which will provide a reading proportional to the current passing through it. Galvanometers are usually quite sensitive, and the current passing through them must be kept to small values, typically in the microamp range, to prevent the galvanometer from burning out.

• A galvanometer can be used to measure currents of any size if they are placed in parallel with a resistor of low resistance. Such a resistor is known as a shunt, because most of the current will flow through it rather than the galvanometer. If a shunt resistor Rs is placed in parallel with a galvanometer of resistance r, the current through the galvanometer, Ig, can be found using the equation:

  Ig = I Rs / (r + Rs)

  where I is the current in the circuit.

• A galvanometer can be used as a voltmeter in a circuit if a resistor of high resistance is placed in series with it. The high resistance, known as a multiplier, limits the current that passes through to the galvanometer. If a galvanometer of resistance r is used to measure a voltage V in a circuit, it will draw off a current of:

  Ig = V / (r + Rm)

  where Rm is the resistance of the multiplier.