The brightness activity

Three resistors, one at a time, in the same circuit. The bulb reports the current without anyone reading a meter.

Materials

A small electric torch bulb, a switch, resistors of 5 Ω, 10 Ω and 20 Ω, connecting wires, two dry cells.

Figure to be added

Figure 19.22 — the circuit for activity 19.5: two cells, a switch and a bulb in a single loop, with a gap between points A and B where each resistor is placed in turn.

Method

Set up the circuit as shown in the figure.
Placing one resistor at a time between A and B, observe how brightly the bulb glows, and record your observations in a table.

Resistor value	Brightness of the bulb
5 Ω	Brightest
10 Ω	Dimmer
20 Ω	Dimmest

Observation

As the resistance increases, the brightness of the bulb decreases. It is clear from this that as the resistance of a circuit increases, the current flowing through it decreases.

A dimmer bulb means a smaller current

The bulb's brightness depends on the current through it, so watching the bulb is a way of watching the current. More resistance, less current, dimmer bulb — which is V = IR happening where you can see it.