The machines have various elaborate mechanisms for gauging a coin’s exact dimensions and weight, channelling bogus ones to the device’s return tray. Coins are also passed through a magnetic field. An individual coin’s own magnetic properties determine its path and speed through the sensors, thus sorting counterfeit from credible. While fraudsters can mimic superficial surface properties of coins, it’s virtually impossible to blend metals to achieve such exacting magnetic properties.
Answered by Gareth Mitchell