Fuses

To prevent fires caused by electricity, many buildings had fuses. they were connected to the circuit by means of a fuse box. A fuse has a thin peice of metal across the middle. Suppose someone were to turn on too many things connected to the same circuit. The wires would become very hot. The tin metal in the fuse would melt, "blowing" the fuse.

A circuit breaker works in the same way - only it flips open to create a gap that can be reset.