Galvanometer (CIE AS Physics)

• A galvanometer is a type of sensitive ammeter used to detect electric current
• It is used in a potentiometer to measure e.m.f between two points in a circuit
• The circuit symbol is recognised by an arrow in a circle:

Galvanometer circuit symbol

The galvanometer circuit symbol is a circle with an arrow which deflects

• A galvanometer is made from a coil of wire wrapped around an iron core that rotates inside a magnetic field:

A galvanometer

A galvanometer contains a coil of wire wrapped around an iron core between magnets

• The arrow represents a needle which deflects depending on the amount of current passing through
  • When the arrow is facing directly upward, there is no current
  • This is called null deflection

• Ohm’s law tells us that the current through a conductor (wire) is directly proportional to the potential difference through it i.e. no p.d means no current flows through the galvanometer
• A galvanometer has a p.d of zero when the potential on one side equals the potential on the other side
• This is at the position at which it is connected on the wire (which varies with the sliding contact) gives a p.d equal to the EMF of the cell connected to the galvanometer
• The cell should be connected such that its potential opposes the potential on the wire i.e. the positive terminal of the power supply faces the positive terminal of the cell:

A null galvanometer

A circuit diagram showing a null galvanometer. The voltage V₁ = V₂

• When the sliding contact moves along the potentiometer wire, you add or remove resistance from/to the external circuit. This changes the potential drop across X and Y
• The location of the sliding point is adjusted until the galvanometer reads zero. This is until the potential difference equals E₂
• The direction of the two e.m.fs oppose each other and there is no current