The galvanometer
- 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
Worked example
A power supply and a cell are compared using the potentiometer circuit shown.
The e.m.f produced by the cell is measured on the potentiometer. The potentiometer wire AB is 150.0 cm long and has a resistance of 2.4 Ω. The power supply has an e.m.f of 5.000 V and the solar cell has an e.m.f of 6.25 mV.
Which resistance R must be used so the galvanometer reads zero when AS = 32.0 cm?
A. 735 Ω B. 451 Ω C. 207 Ω D. 401 Ω
Answer: D
Step 1: List the known quantities
- Length of wire AB = 150.0 cm
- Resistance of wire AB = 2.4 Ω
- E.m.f of the power supply = 5.000 V
- E.m.f of the solar cell = 6.25 mV
Step 2: State the condition for the galvanometer to read zero
- The e.m.f of the cell must be equal to the p.d such that the p.d across the galvanometer is zero
Step 3: State the potential divider equation
Step 4: Determine the resistance R1 and R2
R1 = proportion of resistance of the wire AB =
R2 = resistance of resistor R = R
Step 5: Substitute values into the equation
- The galvanometer reads 0 when the e.m.f of the solar cell is 6.25 mV
- Therefore, Vout must be equated to the e.m.f of the cell
Step 6: Rearrange for the resistance R
Examiner Tip
If you’re unsure as to whether the p.d will increase as the contact slider is moved along the wire, remember p.d is proportional to the length of the wire (from Ohm’s law and the resistivity equation). The longer the length of a wire, the higher the p.d.