Abstract: Apparatus for measuring the mass flow of a fluid comprises a first ionizing electrode which, in use, is located in the path of fluid flow and, when energized, ionizes said fluid. A second substantially cylindrical, hollow collector electrode extends around said first electrode so that, in use, the ions produced by the first electrode migrate to the second electrode. The second electrode includes a plurality of conductive rings which are spaced apart in the direction of fluid flow and insulating material is interposed between adjacent rings, but is arranged so as not to be exposed to the ions migrating to the rings in use. Means is responsive to the amount of ion migration to the various conductive rings for measuring the displacement of the ions by the fluid flowing past the electrodes, the displacement being dependent upon the mass flow of the fluid.

Abstract: A radiation-sensitive switching circuit has a light emitting diode which supplies light to a photo-transistor, the light being interrupted from time to time. When the photo-transistor is illuminated, current builds up and when this current reaches a predetermined value, a trigger circuit changes state. The peak output of the photo-transistor is measured and the trigger circuit is arranged to change state when the output of the device is a set proportion of the peak output, so as to allow for aging of the components.

Abstract: A radiation-sensitive switching circuit includes a light emitting diode which from time to time illuminates a photo-transistor, the photo-transistor serving when its output reaches a predetermined value to operate a trigger circuit. In order to allow for aging of the components, the current flow through the diode is increased when the output from the transistor falls below a known level. Conveniently, this is achieved by having a transistor in parallel with the diode, and turning the transistor off when the output from the photo-transistor becomes too low.