The purpose of the paper is to describe the construction and test of an alternating‐current bolometer which was recently built at the Massachusetts Institute of Technology. Previous theoretical work had indicated that such an instrument would be of value in the measurement of radiation and would possess certain advantages over previous devices used for that purpose. Tests substantiate the predictions of the theory. The a.c. bolometer is found to be rugged, easily operated, not susceptible to vibration and unaffected by spurious thermal e.m.f.'s. The sensitivity obtained with an experimental cell using 0.0016‐inch tungsten wire was 0.068 volt per watt/cm2which was in good agreement with theory. There seems to be no reason why cells made with smaller wires should not give sensitivities of five or ten times this value. Experimental results are also given for bolometer response as a function of current and of time.