A new type of magnetron is described in which the split cylindrical anode is made approximately one-quarter wave in length, the two segments being short-circuited at one end. The anode resonates and acts as a tank circuit. Thus difficulties due to interelectrode capacitance and tube lead inductance are circumvented and a much greater heat radiating area is provided. An output of 20 watts at 3750 megacycles (8 centimeters wavelength) and an efficiency of 22 per cent is obtainable. The theory of the anode tank circuit is developed, and expressions are given for wavelength, internal resistance, and logarithmic decrement.