A Carruthers Drift Indicator in a wooden case for transport, containing the drift indicator and various spare parts including two lead weights. The indicator was designed by J.N. Carruthers of the Ministry of Agriculture and Fisheries in about 1924 in order investigate the currents of the North Sea in support of the fishing industry. It measures residual currents (i.e. currents over and above those caused by tidal flow), by combining features of the Ekman current meter and the Robinson anemometer. In operation, a set of cups rotate in the current and, via a set of gears, release phosphor-bronnze balls from a hopper into a divided chamber. The chamber in which the ball is deposited is dictated by the direction of the current, so the number of balls in each chamber after a particular time records the amount of time the current was along that heading. The hopper can hold thousands of balls, typically using 700-800 per day. It can be operated from the surface or at a chosen depth.

This instrument was described by Carruthers in 1926; see J.N. Carruthers et al. ‘A New Current Measuring Instrument for the Purposes of Fishery Research’, J. Cons. Int. Explor. Mer, (1926) pp.127-39. The instrument was initially tested at the Lowestoft municipal swimming baths, before being used as part of the International Council for the Exploration of the Seas, Southern North Sea, Hydrographic Surveys. These surveys utilized lightships moored in and around the south-east coast of England; from the Strait of Dover to the Humber Estuary. Lightships were used because they maintained a constant position allowing for accurate long-term recordings to be made. This device was later modified following initial trials; these modifications are detailed in J.N. Carruthers, ‘The Water Movements in the Strait of Dover: Exceptional Currents in Winter 1929-30’, J. Cons. Int. Explor. Mer, (1930) pp.167-91. There is evidence of these modifications in the various spare parts contained within box carrying the Drift Meter, some appear unused.