The correct answer is: B. Passive transducers
A passive transducer is a device that converts energy from one form to another without the addition of energy from an external source. Strain gauges, LVDTs, and thermocouples are all examples of passive transducers.
A strain gauge is a device that measures the deformation of an object. It is made up of a thin wire that is bonded to the object. When the object is deformed, the wire stretches or contracts, which changes its resistance. This change in resistance can be measured to determine the amount of deformation.
An LVDT (linear variable differential transformer) is a device that measures linear displacement. It consists of a primary coil, a secondary coil, and a magnetic core. The primary coil is energized with an alternating current, which creates a magnetic field. The magnetic field induces a voltage in the secondary coil. The voltage in the secondary coil is proportional to the displacement of the magnetic core.
A thermocouple is a device that measures temperature. It consists of two dissimilar metals that are joined together. When the temperature of the junction changes, an electric current is generated. The current is proportional to the temperature difference between the two metals.
Active transducers, on the other hand, require an external source of energy to operate. They convert energy from one form to another and also amplify the signal. Examples of active transducers include microphones, loudspeakers, and amplifiers.
Analog transducers produce an output signal that is proportional to the input signal. Digital transducers produce an output signal that is in the form of discrete values.
Primary transducers are transducers that measure the physical quantity directly. Secondary transducers are transducers that measure the output of a primary transducer.