Isothermal calorimeters are usually of the heat conduction type in which the heat produced in the sample is conducted away from the sample to a heat sink to keep the sample temperature essentially constant. Figure 2 shows the principal design of such an instrument. The sample is placed in contact with a heat flow sensor that is in contact with a heat sink. On a parallel heat flow sensor an inert reference sample is placed. Disturbances entering the instrument will influence both the sample and the reference sensors in the same way and as the two heat f low sensors are differentially connected disturbances will cancel out. Heat conduction calorimeters of the kind shown in Fig. 2 have been used