Introduction:
A thermocouple is a temperature sensor that generates a voltage signal based on the temperature difference between two junctions of dissimilar metals. It is widely used in various industries for temperature measurement and control applications.

Working Principle of a Thermocouple:
A thermocouple consists of two wires made of different metals, typically known as the positive and negative legs or the hot and cold junctions. When there is a temperature difference between these two junctions, it causes a temperature-dependent voltage to be generated across the thermocouple.

Output of a Thermocouple:
The output of a thermocouple is a D.C. voltage. This voltage is known as the thermoelectric voltage or the Seebeck voltage. It is generated due to the phenomenon known as the Seebeck effect, which occurs when there is a temperature gradient along the length of the thermocouple.

The magnitude of the thermoelectric voltage depends on the temperature difference between the two junctions and the type of metals used in the thermocouple. Different combinations of metals produce different thermoelectric voltages.

Characteristics of Thermocouple Output:
- Polarity: The thermoelectric voltage has a polarity, which means it has a positive and negative terminal. The polarity depends on the temperature of the hot junction relative to the cold junction.
- Linearity: The thermocouple output is generally linear over a certain temperature range. This linearity allows for accurate temperature measurement and control.
- Temperature Range: Thermocouples can measure a wide range of temperatures, from very low (-200°C) to very high (over 2000°C), depending on the type of thermocouple used.

Measurement and Conversion:
To measure the output voltage of a thermocouple, it is connected to a temperature measuring instrument called a thermocouple meter or a temperature controller. These instruments are designed to measure the thermoelectric voltage and convert it into a corresponding temperature reading.

Conclusion:
In summary, the output of a thermocouple is a D.C. voltage. The magnitude of this voltage is proportional to the temperature difference between the two junctions of the thermocouple.