Why a difference is needed

Two cells and a bulb, wired three different ways. Two of the three leave the bulb dark, and the reason is the point of the whole section.

Materials

Two dry cells, conducting wires, a voltmeter, an ammeter, a bulb.

Figure to be added

Figure 19.14 (a) — the three different ways the two dry cells can be connected to the bulb, each with a voltmeter across the bulb and an ammeter in the circuit.

Figure to be added

Figure 19.14 (b) — the circuit diagram for each of the three cases, showing the cells drawn with their positive and negative terminals marked.

In all three cases the voltmeter is connected to measure the potential difference between the bulb's terminals, and the ammeter to measure the current through the bulb. Build each arrangement, watch whether the bulb lights, and record both readings.

Case	How the cells are joined to the bulb	Potential difference	The bulb
1	The positive terminals of both cells	None	Does not light
2	The negative terminals of both cells	None	Does not light
3	The positive of one and the negative of the other	There is one	Lights

Reading the result

In case 1 both of the bulb's terminals are joined to positive terminals, so there is no potential difference between them, and with no potential difference no current flows. Case 2 does the same thing with two negative terminals and fails for the same reason. Only in case 3, with one terminal of each kind, is there a potential difference across the bulb — and then a current flows and the bulb lights.

The conclusion

For a current to flow through a conductor, a potential difference between its two ends is essential. Two cells are not enough on their own; they have to be arranged so that the two ends of the bulb are at different potentials.