The purpose of the following article is to bring to the attention of the physicist, who is interested in the application of physics to physiology, a typical problem in biophysics. Rein, a physicist at the University of Freiburg, was confronted with the necessity of knowing accurately the blood supply to a certain organ. Because the various standard methods proved inadequate he developed an ingenious method which he calls the thermo‐stromuhr. This method of determining blood flow has been modified in two respects: in the method of measuring the heating energy applied to the blood vessel, and in the method of construction of the diathermy‐thermoelements. A tuned circuit is used which permits the measurement of resistance by the method of substitution. The diathermy‐thermoelements are made from transparent bakelite and the process of construction is fully described. The theory of the thermo‐stromuhr as well as the experimental application is explained in detail.