practice accuracy can be improved by connecting a fixed load such as an incandescent light bulb, adding the device in standby, and using the difference in power consumption. This moves the measurement out of the problematic low-power zone. Radio frequency Instruments with moving coils can be calibrated for direct current or power frequency currents up to a few hundred hertz. At radio frequencies (RF) a common method is a rectifier circuit arranged to respond to current in a transmission line; the system is calibrated for the known circuit impedance. Diode detectors are either directly connected to the source, or used with a sampling system that diverts only a portion of the RF power through the detector. Thermistors and thermocouples are used to measure heat produced by RF power and can be calibrated either directly or by comparison with a known reference source of power. A bolometer power sensor converts incident radio frequency power to heat. The sensor element is maintained at a constant temperature by a small direct current. The reduction in current required to maintain temperature is related to the incident RF power. Instruments of this type are used throughout the RF spectrum and can even measure visible light power. For high-power measurements, a calorimeter directly measures heat produced by RF power. Watthour meters An instrument which measures electrical energy in watt hours is essentially a wattmeter which integrates the power over time (essentially multiplies the power by elapsed time). Digital electronic instruments measure many parameters and can be used where a wattmeter is needed: volts, current in amperes, apparent instantaneous power, actual power, power factor, energy in [k]W·h over a period of time, and cost of electricity consumed. See also Microwave power meter References Further reading External links DC Metering Circuits chapter from Lessons in Electric Circuits series Electrical meters Electricity meters Electronic test equipment