points of an electrically conducting material when there is a temperature difference between those two points. Under open-circuit conditions where there is no internal current flow, the gradient of voltage () is directly proportional to the gradient in temperature (): where is a temperature-dependent material property known as the Seebeck coefficient. The standard measurement configuration shown in the figure shows four temperature regions and thus four voltage contributions: Change from to , in the lower copper wire. Change from to , in the alumel wire. Change from to , in the chromel wire. Change from to , in the upper copper wire. The first and fourth contributions cancel out exactly, because these regions involve the same temperature change and an identical material. As a result, does not influence the measured voltage. The second and third contributions do not cancel, as they involve different materials. The measured voltage turns out to be where and are the Seebeck coefficients of the conductors attached to the positive and negative terminals of the voltmeter, respectively (chromel and alumel in the figure). Characteristic function The thermocouple's behaviour is captured by a characteristic function , which needs only to be consulted at two arguments: In terms of the Seebeck coefficients, the characteristic function is defined by The constant of integration in this indefinite integral has no significance, but is conventionally chosen such that . Thermocouple manufacturers and metrology standards organizations such as NIST provide tables of the function that have been measured and interpolated over a range of temperatures, for particular thermocouple types (see External links section for access to these tables). Reference junction To obtain the desired measurement of , it is not sufficient to just measure . The temperature at the reference junctions must be already known. Two strategies are often used here: "Ice bath" method: The reference junction block is immersed in a semi-frozen bath of distilled water at atmospheric pressure. The precise temperature of the melting point phase transition acts as a natural thermostat, fixing to 0 °C. Reference junction sensor (known as ""): The reference junction block is allowed to vary in temperature, but the temperature is measured at this block using a separate temperature sensor. This secondary measurement is used to compensate for temperature variation at the junction block. The thermocouple junction is often exposed to extreme environments, while the reference junction is often mounted near the instrument's location. Semiconductor