Model 434A is a power meter which bridges the gap between bolometric methods and conventional calorimetric methods while retaining the advantages of both. Bolometric methods of power measurement using the self-balancing principle are fast, but the upper limit of power measurement is quite low (10 to 100 mw). On the other hand, conventional calormetric methods are slow, and are not suitable at powers less than about 1 watt. By combining the self-balancing principle with a calorimetric device, fast response has been obtained over the range from 1 milliwatt to 10 watts.