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A technique is presented for accurately measuring and displaying the magnitude and time function of the exchange and absorption of electrical energy in linear and nonlinear circuit elements such as semiconductor power switches. A novel electronic wattmeter with a wide-band (dc to 10 MHz) linear multiplier is used to observe the instantaneous product of the time-varying voltage and current in an arbitrary load. Experimental data show that the peaks of power dissipation during the turn-on and turn-off phases of a switching power transistor are much greater than the power dissipated during the time intervals of settled current flow. Furthermore, it is verified experimentally that the average power dissipation increases linearly with the transistor pulse-repetition frequency beyond a static (dc) value. The absence of such a frequency-dependent component of power dissipation in switches of resonant-current circuits is also confirmed.