What is a reference coin?

The Reference coin is the coin that is fixed or held by spring tension behind the little sticker that says “slide to replace coin”. This coin is required and must be in place for the comparator to have something to compare to.
You can change your reference coin from copper to zinc, or even a Nickel (when sorting for nickels of better alloy), but no matter what you do if there is no reference coin it will not work properly.