What is a reference coin?
The reference coin is also called a compare coin, sample coin, standard, master, or fixed coin. It is held in the discriminator device also called a comparator. This is the coin all raw coins are measured against electronically to determine if they are to be kept or rejected. If a coin in the raw coins matches the reference coin, it will be accepted, if it is not metalically the same it will be rejected.
Heres a little video that describes the Reference Coin and the Sensitivity Adjustment screw (potentiomenter or Variable Resistor).