A coin dropped into a slot buys you all sorts of goods and services from machines railway tickets, telephone calls, drinks and snacks from a vending machine, copies from a photocopier and, occasionally, a jackpot from a fruit machine.

But before machines will deliver, they scrutinise each coin, subjecting it to a battery of tests. They reject coins of the wrong denomination, foreign coins, deliberate fakes and washers.

Every type of coin throughout the world has its own characteristics. Coins vary in diameter, thickness and weight. And they vary in chemical composition.

In slot machines all these properties are investigated, and only when a coin has taken the right path through a machine can it trigger off the operating mechanism.

A typical slot machine works as follows, although there are many variations. The checking system starts with the slot itself. Coins that are too wide, too thick or too bent will not go in.

Coins that do go through may be tested with a probe to discover if they have a hole in the middle (this detects washers). Genuine coins fall onto a precisely counterbalanced cradle. If the coin is heavy enough, it topples the cradle and is directed onto the runway. If it is too light, it fails to topple the cradle and falls into the reject channel.

An approved coin travels along the runway and past a magnet. Tiny electric currents are set up inside it as it passes through the magnet’s field, causing it to slow down. The amount depends on its composition, since different metals respond to magnetic influence differently.

A coin with the correct composition slows just the right amount and falls off the runway in a trajectory that makes it miss the next obstacle, a deflector. Instead, it hits the separator below at just the right angle to be directed into the ‘accept’ channel. Overweight coins and those less affected by the magnet hit the deflector and rebound on the wrong side of the separator and into the reject channel.