The e.m.f. developed by a thermocouple depends upon:

The electromotive force (e.m.f.) developed by a thermocouple is influenced by several factors. Among these factors, the material used and the temperatures of the hot and cold junctions play a significant role in determining the e.m.f. generated.

Material used:
- The choice of materials used in the thermocouple determines the voltage produced. Different combinations of materials have different temperature-voltage relationships. Each combination produces a unique voltage output for a given temperature difference.
- Different materials have different thermoelectric properties, such as the Seebeck coefficient, which is a measure of the voltage produced per unit temperature difference. The Seebeck coefficient varies for different materials and influences the e.m.f. generated by the thermocouple.

Temperatures of hot and cold junctions:
- The temperature difference between the hot and cold junctions is another critical factor affecting the e.m.f. produced. The greater the temperature difference, the higher the voltage generated by the thermocouple.
- The thermocouple exploits the phenomenon of the Seebeck effect, where a voltage is produced across a junction of dissimilar materials when there is a temperature difference across it. The magnitude of the e.m.f. is directly proportional to the temperature difference.

Other factors:
While the material used and the temperatures of the hot and cold junctions are the primary factors determining the e.m.f. developed by a thermocouple, other factors can also have secondary effects. These factors include:

- Length of wires: The length of the wires connecting the thermocouple to the measuring instrument can introduce additional resistance, which can affect the voltage measurement. However, this effect is generally small and can be compensated for.

- Diameter of wires: The diameter of the wires used in the thermocouple can affect the resistance and hence the voltage measurement. Thicker wires generally have lower resistance and can minimize any potential measurement errors.

- Temperature gradient: The temperature gradient along the length of the thermocouple wires can also have an impact on the e.m.f. generated. A more uniform temperature distribution can help minimize errors and provide more accurate measurements.

In conclusion, while the length of wires, diameter of wires, and temperature gradient can have some influence, the material used and the temperatures of the hot and cold junctions are the primary factors determining the e.m.f. developed by a thermocouple.