From static to a current

Charges resting on a rod do nothing visible. Give them somewhere to go, and they light a bulb on the way.

Materials

A piece of PVC pipe, a piece of polythene, a neon bulb, conducting wires, a stand.

Method

Set up the apparatus as in the figure, connecting the neon bulb with conducting wires. One terminal of the neon bulb must be joined to a wire that is well connected to the earth.
Charge the PVC rod by rubbing it with the polythene.
Touch the charged rod to the terminal of the neon bulb.
Repeat the experiment several times and watch for the neon bulb glowing.

Figure to be added

Figure 19.4 — the PVC rod touching one terminal of a neon bulb, the bulb's other terminal wired down to an earth connection, so the static charge on the rod flows through the bulb and makes it glow.

Observation

The neon bulb glows briefly at the moment the charged rod touches its terminal.

What happened

Static electric charges are stored on the PVC rod that was rubbed with polythene. When the rod touches the bulb's terminal, those stored charges flow away through the conductor. The bulb glows because electric charges are passing through it.

A flow is all a current is

Nothing about the charge changed between the last page and this one. The only new thing is a path — and a charge with somewhere to go is a current.

Why one end has to be earthed

The charge needs somewhere to end up. Without the connection to earth there is no path away from the rod, so nothing flows and the bulb stays dark — the same reason the bird on the wire at the end of this chapter is unharmed.