A CAPACITANCE-WIRE RECORDER FOR SMALL WAVES

Abstract

For some years there has been an increasingly urgent requirement for a satisfactory device for the measurement of small waves such as those in harbour models and wave tanks. Many earlier instruments developed for this purpose used floats, but these suffer from certain practical disadvantages. In particular a large arm is necessary to transmit the motion of the float to the measuring device, which must be well clear of the water surface, and this arm introduces inertia and hence lag into the system. To reduce this lag the float has to be fairly large, and it cannot be regarded as measuring the wave height at a point on the surface. The device described below, in which the capacitance between an insulated copper wire and the surrounding water is measured (figure l), does effectively measure the height at a point on the surface and has given satisfactory results in several applications. Instruments working on this principle are used in many hydraulics laboratories, and a description of those used at the Laboratoire Dauphinois d'Hydraulique has been given by Boudan in "La Houille Blanche", Vol. 8, p. 526 (Aug.-Sept. 1953). The National Institute of Oceanography demonstrated one of these wave recorders at the Conversazione of the Institution of Civil Engineers in June 1952, and it was briefly reported in the July 1952 issue of "The Dock and Harbour Authority". The instrument described below appears to be simpler than others at present in use, and the authors feel that a description of it will be of general interest.