Moving hot zone in aluminium foil

Abstract

order to avoid breaking of the sample. The time interval between each voltage increase was at least 2 min to allow the sample to reach its new thermal equilibrium. Fig. 2 shows that the resistance increases gradually with increasing voltage. This is due to the joule heat generated in the sample. When an a.c. voltage of 4 V was reached a hot zone appeared and the current decreased significantly. However, when the voltage was not lowered, samples broke almost immediately after the hot zone appeared. Fig. 3 shows photographs after the appearance of the hot zone for a sample positioned vertically. As can be seen, the temperature distribution in the hot zone is not homogeneous. A bright spot in the upper part of the bot zone is clearly visible in Fig. 3b, taken after 90 s. Preliminary experiments showed that the hot zone in aluminium wires in a vacuum obtained by a rotary pump did not move. Therefore, the movement of the molten zone in aluminium foils is believed to be related to the convection of air around the molten zone, and not to convection in the melt itself. Due to the convection, the temperature at the top of the hot zone was higher than in the rest of the sample. As a result the resistance of the top region of the bot zone was higher and the voltage concentrated over that region. When the sample was positioned horizontally a hot zone appeared that did not move. This suggests that the above explanation is indeed plausible. Fig. 4 shows that the velocity of the hot zone is not constant. This is believed to be caused by the roughness of the sample edge and inhomogeneities introduced by handling the foil, both of which introduce local changes in the resistance. The