To form a transducer, two uniformly d.c. biased signal detector means are connected in parallel. Each of these signal detector means includes a variable resistor that controls the output frequency of that signal detector means. The variable resistors are located in environments differing only in a parameter for which differential measurement is desired. Such parameters include temperature, thermal absorption, and other variables that affect electrical current flow in the resistors. A non-uniform change in the variable resistors changes the frequency differential existing between the two signal detector means.