The latest generation of slot machines check coins electronically. As soon as a coin is inserted, its conductance – its ability to pass electric current – is tested.
Broadly acceptable coins then pass through a gate and are allowed to run down a ramp and between two magnets. Once again, the speed at which they leave the magnets depends upon their composition. Sets of light-emitting diodes and photosensors measure the coin’s speed. If the values agree with those in the machine’s memory, a gate opens to accept the coin. If not, the coin is rejected. Some machines can be programmed to cope with as many as eight different types of coin.
They can also be programmed to give change. As a coin passes through the checking system its value is identified. When it reaches the end of its journey a microchip releases the appropriate change from stacks of smaller-value coins that are kept in tubes inside the machine.