"The 42A is a wattmeter from Boonton. A Wattmeter is an instrument for measuring the electric power (or the supply rate of electrical energy) in watts of any given circuit. Electromagnetic wattmeters are used for measurement of utility frequency and audio frequency power; other types are required for radio frequency measurements.","Additional Features:","The Boonton 42A RF Microwattmeter provides for 10 nW to 10 mW (7 ranges); Full Scale dBm Ranges of -50, -40, -30, -20, -10, 0 & +10; Solid State, Programmable Instrument; For use with the 41-4A (200 kHz to 7 GHz), 41-4B (200 kHz to 12.4 GHz) or 41-4C (200 kHz to 1 GHz) Power Heads."