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A transverse current traveling-wave tube employing a flat helix and a skew beam has been built and tested both as a forward wave amplifier and as a backward wave oscillator. The tube operates as an amplifier over the frequency range from 1 to 2 kmc with a power output of the order of 30 milliwatts. Tests on the tube indicated a gain vs voltage characteristic markedly different from that obtained with a conventional traveling-wave tube, particularly with respect to the wide range of current and voltage over which a large amount of attenuation in excess of the cold insertion loss of the circuit was observed. The general shapes of the curves of gain vs current and the gain and saturation power level vs frequency are as predicted from theory and display significant differences from calculated curves for an equivalent conventional tube. The nonlinear behavior of the tube, as a function of input signal level, follows the general pattern predicted from qualitative arguments. The output power is constant to within 0.05 db over an input power range of 20 to 30 db. With two signals of different frequencies supplied to the input of the tube, a region of operation was observed in which, with the input amplitude of one signal fixed, the output amplitudes of both signals were independent of the amplitude of the other (variable) input signal.