Calipers and coin gauges: checking a coin's size.

Why size and weight together

Gold is denser than almost every cheap metal. A fake made to the right size comes out light; one made to the right weight comes out too thick or too wide. The maker of the Fisch gauge puts it in numbers: a lead fake the size of a Krugerrand would be 35% lighter, and one of the right weight 54% too thick.

Digital calipers

An ordinary digital caliper reads to 0.01 mm, which is enough. Measure the diameter across several directions, and the thickness at the rim, then compare with the mint's figures:

The U.S. and Canadian mints don't publish thicknesses, so compare a suspect coin with one you know is genuine. Worn older coins can be slightly light; new bullion coins should match closely. Weigh on a scale that reads to 0.01 g; see what scale to buy.

The Fisch gauge

The Fisch is a plastic gauge cut for specific coins, such as the Krugerrand, Gold Eagle, Maple Leaf, Buffalo, Philharmonic, and Sovereign. In one tool it checks four things: a slot for maximum diameter and thickness, a recess for shape, and a balance that tips if the coin is underweight (set slightly below the minted weight to allow for wear). A silver version uses go and no-go slots.

What it can't catch

Tungsten weighs almost exactly what gold does, so a gold-plated tungsten coin or bar can pass size and weight. The Fisch maker sells a separate tool for that. Dealers use resistivity, such as the Sigma verifier, which reads through the coin. See what equipment you need.

Sources

• Fisch Instruments, why it works and how it works.
• U.S. Mint, gold and silver bullion coin specifications; Royal Canadian Mint, 1 ozt Gold Maple Leaf; FNB, Krugerrand specifications; Rand Refinery, products.
• Plansee, tungsten (density).

Equipment: What equipment do you need? · Is my gold coin real? · Krugerrand vs Gold Eagle · All guides

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