Magnetic Scale / Swing Test
Also: Swing test, Lenz test, Diamagnetism test
The magnetic scale exploits the Lenz effect together with the diamagnetic values specific to precious metals in order to tell genuine gold and silver bars or coins from magnetic fakes.
The magnetic scale — also called the swing test or Lenz test — is a non-destructive method for verifying precious metals that relies on the physical principle of Lenz's law. When a strong permanent neodymium magnet is moved at an angle over a genuine gold coin or gold bar (or, conversely, the metal is passed by a magnet), the diamagnetic metal induces an opposing eddy-current brake — the piece tips noticeably more slowly than expected, or a suspended magnet is deflected.
The physical principle
Precious metals such as gold and silver are diamagnetic: they are minutely repelled by magnets and generate measurable eddy currents as a magnetic field passes through them (the Lenz effect). Ferromagnetic or paramagnetic materials used for counterfeits (e.g. tungsten, lead, copper-zinc alloys) behave fundamentally differently — either they are attracted or the eddy-current braking is markedly weaker.
Eddy-current braking force ∝ σ · B² · v · V
σ = electrical conductivity (S/m)
B = flux density of the magnet (T)
v = relative velocity (m/s)
V = sample volume (m³)
The braking force therefore depends on conductivity, magnetic field strength and sample volume — all material-specific quantities that a counterfeiter can scarcely fake all at once.
How the swing test is carried out
- Set-up: a sloped ramp (approx. 45°) of non-magnetic material (wood, acrylic) and a strong neodymium magnet (at least N42) are needed.
- Establish a reference: let a verified original coin (e.g. a Krugerrand or Vienna Philharmonic) slide down the ramp and time it.
- Test the candidate: run the suspect coin or bar under identical conditions.
- Evaluation: significantly faster sliding, or attraction to the magnet, points to a fake; a steady delay indicates genuine precious metal.
Limits of the method
The magnetic scale is no cure-all. Gold-plated tungsten fakes have almost the same density as real gold (19.25 vs. 19.32 g/cm³), and because tungsten is weakly paramagnetic, the eddy-current braking in the swing test comes out similarly small to that of genuine gold. A swing test alone cannot reliably distinguish the two here. For dependable results it should therefore always be combined with the density test after Archimedes and X-ray fluorescence analysis. A weight and dimension check using the Coin Weight Check is also sensible.
In brief
The swing test is a quick, low-cost entry point into precious-metal authentication that reliably exposes ferromagnetic fakes — but for high-quality tungsten imitations it needs supplementary methods such as density measurement or ultrasound.