How does a temperature transmitter work?

Transmitters offer the following advantages: Sensor signals are sensitive: a thermocouple delivers only a few millivolts; a Pt100 changes its resistance only slightly with temperature. Over longer cable lengths, these signals would easily be distorted. Coupling to a transmitter eliminates these disadvantages. 

Standardisation: Industrial measuring and control devices are often designed for standard signals (4–20 mA is common worldwide). 

Interference immunity: Current signals are less susceptible to electromagnetic interference and voltage losses than raw signals.