The paper presents the results of an investigation of unduloid formation on hard-drawn and soft silver wires with diameters between 0-16 and 1 -0 mm. It has been established that unduloids only form on harddrawn wires. Consequently, unless wire surface hardness is taken into account, the existing theories about the surface melting of current-overloaded wires may be incorrect. Furthermore, experiments are reported on hard-drawn and soft wires heated with a gas flame where a similar formation of unduloids only occurred on the hard-drawn wires. It has been found that, for both soft and hard-drawn wires, there is a close similarity between their behaviour when heated by either a gas flame or. an electric current.