Question

which device convert message signal into equivalent electrical signal

Answer 1

Answer :- transducer

Explanation :- you mentioned , " message signal " it is not electrical signal . Message signal is another type of non- electrical signal which represents message. So, the device which converts non- electrical signal to electric signal is transducer . Transducer is not only used for converting non- electrical signal to electrical signal , it is basically used for converting signal one form to another form.