A New High-Frequency Current Standard

A short-circuited-ring electrodynamic ammeter is described. The short-circuited ring is supported by a fine quartz fiber and is suspended midway between the inner and outer conductors of a coaxial transmission line. The current on the line is found by measuring the torque exerted against the ring by the current on the coaxial line. A new technique is given for determining the relationship between the torque on the ring and the current on the transmission line, and is compared with the technique used in the past. The new method is an application of the resonator action theorem used by Cullen for calibrating a torque-operated microwave wattmeter. The major weakness in the previous evaluations of the torque-current relation is discussed. The ammeter is useful for measuring current from 1 to 100 amperes over the frequency range 1 MHz to 1 GHz, with an uncertainty of the order of 0.5 percent.