Introducing electric current

Once your students understand how circuits can be set up, it is time to introduce them to the idea of the lamp as an informal ammeter. A fully-lit lamp connected to one cell can be said to indicate ‘one lamp’s worth of current’ (whatever current really is). Two lamps lit by one cell will each have less than ‘one lamp’s worth’ of current through them. One lamp connected to two cells will have more than ‘one lamp’s worth’ of current through it.

Some students may wish to know more about the electric current. At an introductory level, discussion along the following lines might be suitable.

You cannot see an electric current, or hear it, or know about it, by anything except by what it does.

How do you know that your Uncle George has a bad temper? Because he talks very crossly when you annoy him. You only know he has a bad temper by its effects. You cannot see a warning light on his head labelled ‘bad temper’ or a tribe of little demons dancing in his stomach to keep him irritable.

Some of you may have heard that when there is an electric current there are little electrons running along in the wire but you cannot see them any more than you can see the demons in Uncle George’s stomach. If we behave as good scientists and stick to the evidence, we can say that we see a lamp that's lit and a wire that’s hot. From this we infer there is an electric current.