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Morgan Dollar — ping test & specs

The Morgan Dollar (1878–1921) is a classic 90% silver US dollar. Its copper alloy and worn surfaces make its ring vary more between specimens than modern bullion, so a wider tolerance is used.

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Opens the free tester with this coin preselected — audio never leaves your device.

Official specifications

Weight26.73 g
Diameter38.1 mm
Thickness2.4 mm
Composition.900 silver (90%)

Source: United States Mint. Coin Pinger checks measured weight and diameter against these values.

Expected resonance range

High confidence*
ModeExpected ringRecommended variance
c0d23.86–4.82 kHz±11.1%
c0d38.93–11.07 kHz±10.7%
c0d415.56–19.29 kHz±10.7%

A genuine Morgan Dollar rings within these ranges at its c0d2/c0d3/c0d4 vibration modes, fixed by its size and metal. The recommended variance is how far a genuine specimen's ring can sit from each center — built up in quadrature from real manufacturing tolerance and temperature (under 1%), phone-mic capture reproducibility (≈4%), an extra allowance on the fundamental for minting-asymmetry splitting, and a per-metal model-uncertainty term (largest for .9999 gold, whose modulus is least certain). This coin's band is widened further because circulated 90%-silver classics lose mass and edge crispness with wear, spreading their ring. The higher modes also carry a small thick-plate centre correction, since a real coin is not infinitely thin. The high-confidence tag reflects how well-established this metal's constants are: Computed from well-established silver elastic constants and cross-checked against same-size sibling coins. These are guidance bands, not exact values — see the methodology for the exact formula, or calibrate your own coin in the app for a specimen-exact reference and tighten each peak per coin.

* These are computed guidance ranges. Certain your Morgan Dollar is genuine but it won't match? Send us its ring — we'll dig in and improve the reference.

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How to ping test a Morgan Dollar

  1. Hold — grip the coin by its center (a nodal point) in a coin ping-test holder so the rim rings freely; a fingertip at the center is a last resort.
  2. Tap — strike the edge with a striker near the microphone.
  3. Read the verdict — MATCH or NO MATCH against the peaks above, plus weight and size checks.
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The physics of the Morgan Dollar ping

Those 3 peaks aren't random. When you strike the coin it flexes as a free disc in fixed patterns called vibration modes — 0 nodal circles with 2, 3 and 4 nodal diameters, written c0d2 / c0d3 / c0d4. Each pattern rings at one frequency.

Where those frequencies land is set by the coin's geometry and metal. For a thin disc, frequency ∝ (thickness ÷ diameter²) × √(stiffness ÷ density). This Morgan Dollar is 38.1 mm across and about 2.4 mm thick in .900 silver (90%), which puts its fundamental (c0d2) near 4340 Hz.

A perfect disc's modes sit near 1 : 2.3 : 3.9; this coin measures 1 : 2.30 : 4.01. That fixed spacing is the fingerprint of a genuine struck disc — Coin Pinger rejects taps that don't fit it (keys, glass, a coin lying on a hard surface).

And it's why a fake can't hide: change the metal and √(E/ρ) changes, so every mode moves. Struck in lead or a plated base metal instead, the ring would land far lower and die out with a dull, short thud — its c0d2 would land near ≈1900 Hz instead of 4340 Hz. Density can fool a scale; the ring can't be faked without the right metal.

We publish this as a High confidence reference. Computed from well-established silver elastic constants and cross-checked against same-size sibling coins.

Free-plate modes per A. W. Leissa, Vibration of Plates, NASA SP-160 (1969, public domain). See how we measure them →
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