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A new electron beam technique for measuring microwave power flow, either cw or pulse, in waveguides is described. This technique consists of accelerating an electron beam transversely through an evacuated section of waveguide carrying power in the TE10 mode. The transit time of the electrons is adjusted to a value which gives maximum interaction of the field in the guide with the electrons, i.e., electrons gain maximum energy. The energy gained by the electrons is measured by means of a dc stopping potential which can be related to the field. Power is then calculated from the Poynting vector. The instrument takes the form of a sealed-off vacuum tube having a short section of waveguide as part of the tube. The ends of the waveguide may be sealed using known window techniques. The theory for the ideal case is presented and then means of correcting for the various perturbations present in an actual tube are discussed. The theory and the corrections are verified by experiment. The technique appears to have definite value for monitoring or measuring high- and medium-level cw or pulse-power flow. Theoretically the device is self-calibrating and therefore might make a good primary standard. Its suitability as a primary standard is under further investigation.