Magnetic Scale / Slide Test

The magnetic scale — also known as the slide test or Lenz test — is a non-destructive method for the authenticity testing of precious metals, based on the physical principle of Lenz's law. When a strong neodymium permanent magnet is passed at an angle over a genuine gold coin or gold bar (or, conversely, the metal is moved past a magnet), the diamagnetic metal induces an opposing eddy-current brake — the piece tips noticeably more slowly than expected, or a hanging magnet is deflected.

Physical principle

Noble metals such as gold and silver are diamagnetic: they are minimally repelled by magnets and generate measurable eddy currents when a magnetic field passes through (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 brake is significantly weaker.

Eddy-current braking force ∝ σ · B² · v · V
σ = electrical conductivity (S/m)
B = flux density of the magnet (T)
v = relative speed (m/s)
V = sample volume (m³)

The braking force therefore depends on conductivity, magnetic field strength and sample volume — all material-specific quantities that counterfeiters can hardly falsify simultaneously.

Procedure of the slide test

1. Set-up: an inclined ramp (approx. 45°) made of non-magnetic material (wood, acrylic) and a strong neodymium magnet (at least N42) are needed.
2. Establish a reference: let a verified original coin (e.g. a Krugerrand or Vienna Philharmonic) slide down the ramp and measure the time.
3. Test the sample: test the suspect coin or bar under the same conditions.
4. Evaluation: significantly faster sliding or attraction to the magnet indicates a counterfeit; a steady deceleration speaks for genuine precious metal.

Limits of the method

The magnetic scale is not a cure-all. Tungsten counterfeits with gold plating have almost the same density as genuine gold (19.25 vs. 19.32 g/cm³) — and since tungsten is weakly paramagnetic, the eddy-current brake in the slide test turns out similarly small as with genuine gold. Here, a slide test alone cannot reliably distinguish. For robust results it should therefore always be combined with density measurement by Archimedes and X-ray fluorescence analysis. A weight and dimension check via the coin weight checker is likewise sensible.

In brief

The slide test is a quick, low-cost first step into the authenticity testing of precious metals, which reliably exposes ferromagnetic counterfeits — for high-quality tungsten imitations, however, supplementary methods such as density measurement or ultrasound are needed.