To make my question simple and easy to understand I am giving an example below. I have an RTD (platinum 100) which change resistance with increase in temperature, for checking that rtd is showing actual temperature I can check the ohms by connecting a multimeter and change its setting to ohms and it will show us the resistance of rtd. now if we connect the rtd to a temperature transmitter or controller of which input is rtd's resistance and output is mA and we try to check the resistance of rtd while connected to transmitter/controller the output of transmitter rises to maximum which is 20mA. I want to know why is this.
I know we should not do like this but I just want to know for learning purpose.