Speed Indicator or Tachometer

Speed Indicator or Tachometer; Hedge's Vortex Speed Indicator, 1885-1890

This instrument closely resembles the velocimeter of Mr. J. Ramsbottom, in which a closed glass tube filled with oil is rotated on its vertical axis, and the resulting depression of the oil, varying as the square of the speed, is measured on a graduated scale (see Railway Locomotive Section). Mr. Killingworth Hedges' instrument, patented in 1885, is an improvement in that the depression of the parabolic cavity is directly proportional to the speed, so that the graduations of the scale are uniform and the range of the instrument is greatly increased. The difference is due to the top of the tube being closed by a flat cover and a much larger proportion of the tube being filled with liquid. When in use the diameter of the parabolic air cavity at the top is less than the diameter of the tube, and varies with the speed in such a manner that the vertex of the paraboloid sinks proportionately with the increase in the speed. A sliding sighting bar is attached to the case to assist in accurately determining the position of the vertex, the reading being taken on the fixed scale.