A method is described in which the ratio of the heat capacities of two samples is measured with an accuracy which approaches 1:104 between 20K and 200K, but which decreases to 1:103 at 1.5K and 300K. The resolution of the system is very much higher than this, and may approach 1:106 over a limited temperature range. The system is extremely simple to operate being the thermal analogue of a Wheatstone bridge, and measurements may be taken rapidly at closely spaced temperature intervals. The construction of the calorimeter, which contains several new features, is also described.