please explain to me what is e.m.f.

e.m.f. stands for electromotive force.
It is the work done per unit charge (Joules per Coulomb) by an electric field when electric charge goes from point A to point B. It is measured in Volts.
1 Volt = 1 Joule/Coulomb

If A and B are points in a circuit, the emf is the voltage difference between those points.

ok, and it is used in thermocouple thermometers right?

Yes, that's correct! Thermocouple thermometers utilize the concept of electromotive force (EMF) to measure temperature.

In a thermocouple thermometer, two different metals are joined together to form a junction. When this junction is subjected to a temperature gradient, an EMF is generated due to the Seebeck effect.

The Seebeck effect is the phenomenon where a voltage difference is created across a junction of two dissimilar metals when there is a temperature difference across the junction. This voltage difference is directly proportional to the temperature difference.

By measuring the EMF produced by the thermocouple and knowing the relationship between EMF and temperature, the temperature at the junction can be determined. This allows thermocouple thermometers to accurately measure temperature in a wide range of applications, including industrial processes, scientific experiments, and even in everyday use.