This paper describes the principles and development of an ammeter which has been made for measuring current of very high frequencies, such as 30 000 kilocycles per sec. The instrument is of the dynamometer type and has a geometrical form for which all the changes of current distribution can be calculated. The correcting factors approach a limiting value and do not increase contruments. A special feature of the design is that an instrument can be made to carry unshunted a current of any magnitude. This removes all uncertainty about shunting ratio and so the instrument should be more reliable than one in which the current is supposed to divide evenly among a number of resistance wires in parallel.Methods are described for checking the calculated correction factors and there are also some calibrations at frequencies round about 1000 kilocycles per sec. In addition, measurements are described of the high-frequency resistance and of the inductance and terminal capacity.