A resistor is a device of known resistance used to regulate current and voltage in a circuit; it consists of a frame, a wire or conducting layer of high resistivity, and a coating. The resistance of a resistor cannot be changed. When resistance must be changed smoothly, a rheostat is used: nickelin or nichrome wire is wound on a ceramic cylinder, and when the slider moves along the turns the length of wire in the circuit changes, and with it the resistance and the current. A rheostat is connected in series with the consumer. A potentiometer is a special way of connecting a rheostat to regulate voltage; round potentiometers are used in radios and televisions to set the volume.
Worked examples
A nickelin wire for a rheostat: 10 m long, S = 0.4 mm². R = 0.4 · 10 : 0.4 = 10 Ω. If the slider puts 3/10 of the wire into the circuit, the resistance is 3 Ω.
If the slider is moved so that a longer piece of wire is in the circuit, the resistance grows, the current falls and the lamp glows more dimly.
Class activity
With an adult: build a simple rheostat from a 3–4.5 V battery, a small lamp and a long (15–20 cm) thick graphite pencil lead: connect one end of the lead with a wire and slide the other wire end along it; watch the brightness change. Low-voltage battery only, no mains.
Practice
1
What is a rheostat for?
To change the resistance in a circuit smoothly, and with it the current.
2
A rheostat has total resistance 10 Ω. The slider puts in 40 % of the wire. What is the resistance in ohms?
4
3
How does a potentiometer differ from a rheostat?
A rheostat is connected to regulate current, a potentiometer to regulate voltage.
4
Why is the wire of a resistor made of a material with high resistivity?
The needed large resistance can be got from a short wire, so the device stays compact.