Magnetic Scale / Swing Test
Also: Swing Test, Lenz Test, Diamagnetism Test
The magnetic scale uses the Lenz effect of precious-metal-specific diamagnetic values to distinguish genuine gold and silver bars or coins from magnetic forgeries.
The magnetic scale — also known as the swing test or Lenz test — is a non-destructive method for authenticating precious metals, based on the physical principle of Lenz's law. When a strong neodymium permanent magnet is moved at an angle over a genuine gold coin or gold bar (or the metal is moved past a magnet), the diamagnetic metal induces an opposing eddy-current brake — the piece tilts noticeably more slowly than expected, or a suspended magnet deflects away.
Physical Principle
Noble metals such as gold and silver are diamagnetic: they are minimally repelled by magnets and generate measurable eddy currents as a magnetic field passes through them (Lenz effect). Ferromagnetic or paramagnetic materials used in forgeries (e.g. tungsten, lead, copper-zinc alloys) behave in a fundamentally different way — they are either attracted or the eddy-current braking force is considerably weaker.
Eddy-current braking force ∝ σ · B² · v · V
σ = electrical conductivity (S/m)
B = magnetic flux density (T)
v = relative velocity (m/s)
V = sample volume (m³)
The braking force therefore depends on conductivity, magnetic flux density, and sample volume — all material-specific quantities that forgers can hardly replicate simultaneously.
Performing the Swing Test
- Setup: A non-magnetic inclined ramp (approx. 45°) made of wood or acrylic and a strong neodymium magnet (at least N42) are required.
- Establish a reference: Let a verified genuine coin (e.g. Krugerrand or Vienna Philharmonic) slide down the ramp and measure the time taken.
- Test the suspect piece: Test the suspect coin or bar under identical conditions.
- Assessment: Significantly faster sliding or attraction to the magnet indicates a forgery; uniform deceleration is consistent with genuine precious metal.
Limitations of the Method
The magnetic scale is no cure-all. Tungsten forgeries with gold plating have an almost identical density to genuine gold (19.25 vs. 19.32 g/cm³) — and since tungsten is weakly paramagnetic, the eddy-current braking force in the swing test is similarly low to that of genuine gold. In this case a swing test alone cannot reliably distinguish the two. For reliable results it should therefore always be combined with the density test (Archimedes method) and X-ray fluorescence analysis. A weight and dimension check using the Coin Weight Checker is also advisable.
In Brief
The swing test is a fast, low-cost first step in precious metal authentication that reliably exposes ferromagnetic forgeries — for high-quality tungsten imitations, however, complementary methods such as density measurement or ultrasound are required.