Assuming you mean a J-Type thermocouple...
A thermocouple is a junction between two different metals that produces a voltage related to a temperature difference. The voltage produced by thermocouples is very small and non-linear.
A J-Type thermocouple is iron-constantan, and has a range of about 200-1400°F (95-760°C).
The Iron is the positive (+) conductor and has a white insulator.
The Constantan is the negative conductor (-) and has a red insulator.