A purpose build device was designed to ensure a standardised gap between the electrodes regardless of their thicknesses consisting of a plastic tube (length 90 mm, i.d. 70 mm, wall thickness 2 mm) and a press to keep the electrodes in position (Fig. S). A slot was cut lengthwise in the top of the tube to allow insertion of a thermocouple probe and an optical fibre (Luxtron, Santa Clara, USA) at the measuring locations. The tube was held in position with two jubilee clips which also served to hold the temperature sensors in position. The optical fibre was used to measure electrode surface temperature at the centre point, while at the centre point of the sample temperature was measured using a thermocouple probe connected to a data logger as described above. Temperature readings for both sensors were validated as described by Zeil et al. (2009a).