Judge each statement against standard transducer theory.

A — "Magnetostrictive transducer operation depends on the change of permeability of a magnetic material when strained." TRUE. This is the Villari effect (inverse magnetostriction): applying mechanical stress to a ferromagnetic material such as nickel or Terfenol-D changes its permeability, which changes the inductance of a surrounding coil and hence gives an electrical output. The forward effect — a magnetic field producing strain (Joule magnetostriction) — is what drives ultrasonic transducers and sonar projectors.

B — "Potentiometer is an active transducer." FALSE. The classification is by energy source: an active (self-generating) transducer produces its own output energy — thermocouple, piezoelectric crystal, photovoltaic cell, tachogenerator. A passive transducer merely varies a circuit parameter and needs external excitation — resistance (potentiometer, strain gauge, RTD, thermistor), capacitance, or inductance (LVDT). A potentiometer is a variable resistance and needs a supply, so it is passive.

C — "Turbine flow meters are volumetric flow meters." TRUE. The rotor speed is proportional to the average fluid velocity, and multiplying by the fixed bore area gives volume per unit time:

\(Q = A v \propto N\)

where N is the rotational speed picked up by a magnetic sensor. To obtain mass flow you must additionally know the density (a Coriolis meter measures mass flow directly).

D — "Any metal can be a thermistor." FALSE. A thermistor is made from sintered semiconductor metal oxides (of manganese, nickel, cobalt, iron), which give a large negative temperature coefficient and a strongly non-linear response. Metals behave in the opposite way — a small, nearly linear positive coefficient — which is what makes them suitable for RTDs (platinum Pt-100), not thermistors.

E — "Thermocouples are used in high accuracy measurements." FALSE as keyed. Thermocouples are prized for their very wide range, ruggedness, small size and fast response, but their millivolt-level output, non-linearity and need for cold-junction compensation make them less accurate than a platinum RTD, which is the standard choice where accuracy matters.

Hence, the correct answer is A and C only.