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A bolometer element with a positive temperature coefficient of resistance, used to detect and measure power at radio, microwave, infrared, and optical frequencies. The temperature of the barretter increases when electromagnetic energy is absorbed. Barretters are made of metal; therefore, the electrical resistance increases when the temperature increases. The resulting resistance change of the barretter is measured by using direct-current or low-frequency instruments. See Bolometer
The barretter resistance is selected to absorb most of the power when the barretter is mounted as a termination in a waveguide or coaxial transmission line. A barretter can be made to detect power at optical and infrared frequencies by using a very thin metal ribbon blackened to absorb light.
Barretters with less sensitivity and accuracy for use at radio frequencies can be made by using low-current fuses made with fine wires.
A meter can be made to measure high-frequency signal amplitudes using a barretter. The temperature and hence the resistance of a barretter can change at audio-frequency rates, but the time constant of a barretter is too great for the resistance to vary at radio-frequency rates. A radio- or microwave-frequency current modulated at a low frequency will cause the barretter resistance to follow the low-frequency signal. If a direct-current voltage is applied to the barretter while the modulated radio-frequency current is also applied, the varying resistance will produce a current which follows the modulation. The low-frequency current can be coupled to the input of an audio amplifier tuned to the modulation frequency by using an audio transformer. The output of the audio amplifier may be rectified to drive a direct-current meter. The meter then indicates the relative amplitude of the radio-frequency or microwave signal.