Potentiometer
A potentiometer measures an emf without drawing current: the potential along its uniform wire grows in proportion to length. If two cells balance at 60 cm and 40 cm, E₁/E₂ = l₁/l₂ = 1.5. With a 2 V driver across a 10 Ω wire, 60 cm (6 Ω) carries 1.2 V.
The wire as two resistors
Tap a resistor to choose it, then change its value or add another in series or in parallel with it. The glowing dots are the current: faster where more flows. Brighter resistors turn more power into heat.
| R | I | V | Power | |
|---|---|---|---|---|
| R₁ | 6 Ω | 0.2 A | 1.2 V | 0.24 W |
| R₂ | 4 Ω | 0.2 A | 0.8 V | 0.16 W |
Step by step
- All in series: Req = 6 + 4 = 10 Ω
- Current from the cell: I = V / Req = 2 / 10 = 0.2 A
- Drops in series: R₁: 0.2 × 6 = 1.2 V, R₂: 0.2 × 4 = 0.8 V.
- Power: P = EI = 2 × 0.2 = 0.4 W.
Why it works
- A steady current from the driver cell flows through the uniform wire AB, so the potential drop per unit length, k, is the same everywhere.
- The potential across a length l is V = kl.
- A cell E₁ joined through a galvanometer to a jockey J balances when E₁ = kl₁. At that point no current flows from the cell.
- With a second cell balanced at l₂, E₁/E₂ = l₁/l₂.
- For internal resistance, balance the cell open (l₁) and with a resistance R across it (l₂): r = R(l₁ - l₂)/l₂.
Potentiometer: quick answers
Why is a potentiometer better than a voltmeter for measuring emf?
At balance it draws no current from the cell, so it reads the true emf. A voltmeter draws a little current and reads the slightly lower terminal voltage.
What is the symbol and circuit of a potentiometer?
In a circuit it is drawn as a long resistance wire AB with a sliding contact, the jockey J, shown as an arrow touching it. The driver cell is across A and B.
What if no balance point can be found?
The emf being measured is larger than the fall of potential across the whole wire. Use a driver cell of higher emf, or check that the + terminals are joined at the same end A.