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What is Thermocouple Wire?

The wires are insulated. This prevents the wires touching except at the tip. The ability of the insulation to withstand temperature is often the limiting factor in working out the highest temperature at which a thermocouple can be used.
The wires are usually welded together at the tip. Any kind of contact at the tip will create a thermocouple. Soldering is possible, as is mechanical crimping, but welding produces the smallest and most robust tips.
Standard combinations of wires are used. Many different materials can be used in the manufacture of thermocouples. These include platinum and platinum-rhodium alloys for use up to 1600 °C; cheaper, mainly nickel-based alloys for industrial use up to 1200 °C; and refractory alloys up to and beyond 2000 °C. Standardised letter-designated thermocouple types (eg Types K, N, T, B, R, S) are manufactured to meet the specifications of IEC 584 (BS EN 60584). This standard gives tables and equations for the EMF (or voltage) generated at every degree in the range, and the tolerances with which they should be manufactured. Thus, for example, a K-type thermocouple has wires made from carefully ed alloys of Nickel-Chromium and Nickel-Aluminium, for which the EMF at 1000 °C should be 41.276 mV, within about 0.16 mV. Summaries of the tables can be found in many reference books, such as (Kaye and Laby, Tables of physical and chemical constants).
· Cold junction compensation. Because the voltage is generated along the wires where there are temperature gradients (see below), it follows that the temperature of the ´free ends´ of the wires is important and must be controlled. The ´reference´ or ´cold´ junctions are commonly held at 0 °C, using melting ice for best accuracy, but in most digital instruments the cold junction effect is compensated by sensing the reference temperature and adding the appropriate voltage to the signal, thereby allowing digital multi-meters to display the temperature directly.