Apparatus is described which is suitable for controlling differential pressures in the range 0-1·5 in water gauge but which could be modified for higher pressures. Changes in the difference in liquid level in a manometer are converted to electrical signals by the displacement of a float to which is attached the core of a linear variable differential transformer. The out-of-balance signal is used to operate either a proportional or an on-off control, the latter being used in the application described.