Ferromagnetism in ancient copper-based coinage

Abstract

WHEN extensive forgery of silver coinage has taken place in the past, a small permanent magnet was a well known technique for distinguishing between counterfeit nickel and genuine silver coins. The accidental discovery of ferromagnetism in undisputably genuine ancient Kushan copper coins by Nisbet during a routine check with a permanent magnet was, however, totally unexpected. Although we have found very few coins possessed a magnetic moment sufficiently large for the coin to be lifted by the permanent magnet, subsequent examination has revealed that several coins possess a spontaneous remanence magnetisation which can be detected with a simple deflection magnetometer. Quantitative measurements of the room temperature magnetisation have been performed in fields up to 5 × 10⁵ A m⁻¹using a Faraday balance magnetometer. To our knowledge they form the first systematic study of the magnetic properties of ancient coinage.