The general theory of the deformation of a bimetallic disk with a small initial curvature due to temperature changes, is developed and applied to the case of a disk initially in the form of a spherical cap. It is shown that if the central rise of the disk is initially greater than about twice the total thickness, a position of instability will occur on both heating and cooling. An expression is derived for the mean temperature which a disk of this type will maintain when used as the control element of a thermostat, and results are obtained for the temperature variation about this mean value.