Sigmascope Eddy Current Tester

The sigmascope is a device for measuring the electrical conductivity of a metal (which is referred to by the Greek letter sigma). It is one of a number of non-destructive testing (NDT) techniques used for detecting counterfeits. Conductivity is the reciprocal of resistivity. Most people who have been taught a little physics in school are familiar with the concept of electrical resistance of an object such as a piece of wire – doubling the length of the wire will double its resistance. But resistivity is an indicator of a metal’s specific electrical resistance regardless of its physical form. There is a similar relationship between electrical conductivity and electrical conductance.

The clever thing about the sigmascope is that it measures conductivity by means of induced eddy currents rather than by simply measuring the electrical current flowing across an object with a voltage applied across it. The probe of the instrument (shown above being applied to a coin under test) generates an alternating magnetic field by means of a coil. This generates varying eddy currents within the object being tested, which in turn affect the magnetic flux within it. The probe detects changes in the magnetic flux, which can be converted into a measure of conductivity of the metal.

The usefulness of this instrument lies in the fact that the electrical conductivity of gold is significantly affected by many parameters that are of interest. These include the presence of alloying elements, inclusions or porosity and, perhaps most relevantly, foreign bodies.

A very informative presentation* on the principles involved in and the use of the eddy current testing of gold was given at LBMA’s 2015 Assaying and Refining Conference.

* Analyses of the Chemical and Structural Integrity of Gold Bullion Bars by Eddy Current Probes – Quentin Bochud, Metalor Technologies, and Daniel Sutter, Helmut-Fischer.