The thermo-emf can be related to the higher temperature for given values of (the reference temperature) as

or read off from calibration charts. Here is the absolute temperature in units of Kelvin. Values of , depend on the choice of the reference temperature. Commonly C and the reference junction is kept in an ice-water mixture. For the ice point as a reference, typical values of and are 1/50 mV/K and . Since thermo-emfs tend to be small, the voltmeter employed for this purpose must have the necessary resolution and accuracy. The thermocouple produces a voltage output in response to a temperature difference and classifies as an active transducer. Pairs of metals/alloys normally used in thermocouples are chromel/alumel, copper/constantan and iron/constantan. The bead that is formed at the junction should be clean, uniform and free of oxides. Normally every new thermocouple that is formed must be tested against an ideal reference thermocouple. The bead size can be increased by using wires of larger diameter to damp temperature fluctuations and reduced if transients need to be followed closely.