Magnetic Balance / Slide Test
Also: Slide test, Lenz test, Diamagnetism test
The magnetic balance uses the Lenz effect of the metal-specific diamagnetism values to distinguish genuine gold and silver bars or coins from magnetic fakes.
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The magnetic balance - also called 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
Precious 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 (Lenz effect). Ferromagnetic or paramagnetic materials used for fakes (e.g. tungsten, lead, copper-zinc alloys) behave fundamentally differently - they are either attracted or the eddy-current brake is much 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 counterfeiters can scarcely falsify simultaneously.
Procedure of the slide test
- Setup: a sloped ramp (approx. 45°) of non-magnetic material (wood, acrylic) and a strong neodymium magnet (at least N42) are required.
- Establish a reference: let a verified original coin (e.g. Krugerrand or Vienna Philharmonic) slide down the ramp and measure the time.
- Apply the test piece: test the suspect coin or bar under the same conditions.
- Assessment: significantly faster sliding or attraction to the magnet indicates a fake; a uniform delay speaks for genuine precious metal.
Limits of the method
The magnetic balance is not a panacea. Tungsten fakes with gold plating have a density almost identical to 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 to that of genuine gold. Here a slide test alone cannot reliably distinguish. For robust results it should therefore always be combined with the 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 entry point into the authenticity testing of precious metals, reliably exposing ferromagnetic fakes - but for high-quality tungsten imitations it needs supplementary methods such as density measurement or ultrasound.