Which of the following is a type of variable transducer?

A variable transducer is a device that converts a physical quantity into an electrical signal that varies with the quantity being measured. Among the options provided, an optical pyrometer qualifies as a variable transducer because it detects temperature based on the intensity of radiation emitted by an object. This measurement results in a varying electrical signal corresponding to changes in temperature.

An optical pyrometer operates by interpreting the color or brightness of the light emitted by a hot object and converting this optical measurement into an electrical output that varies with temperature. This functionality makes it a representative example of a variable transducer.

In contrast, flow meters, pressure gauges, and thermocouples operate differently. While they are also devices used for measurement, they can be classified into more specific categories, including constant or secondary-output devices, depending on the context of their use. Flow meters often provide a direct reading without necessarily converting into a variable signal in the same manner as a transducer. Similarly, pressure gauges typically give a direct indication of pressure rather than producing a variable output signal in the form needed to classify them as variable transducers. Although thermocouples do convert temperature differences into voltage differentials, they are often categorized as sensing elements rather than transducers in the classic sense.

Therefore,