Furnace temperatures are usually measured by thermocouples. Thermocouples work on the principle that differing metals generate a voltage different between them proportional to the temperature. The temperature measured is that at the tip of the thermocouple where the two different metals meet. It must always be remembered where the thermocouple is situated. A thermocouple in a crucible furnace will give a much higher reading that a thermocouple inside the metal crucible. A thermocouple in the roof of a pottery kiln will give a higher reading that the one on the floor, due to the effect of convection. A thermocouple a short distance into a negative pressure furnace can be cooled drastically by air sucked in over through its holes in the shell.