CURRENT METER, an instrument for measuring the velocity of the flow in rivers and streams. The current meter used by the United States Geological Survey consists of two essen tial parts: (1) a wheel which is so arranged that the flowing water shall turn it steadily, and (2) a device for recording the number of such revolutions. The earlier forms of meters were mounted on floats and measured only the sur face velocity. This was soon proved inade quate, and was improved so that measurement at any depth may be made. The current meters in present use are of two distinct types, direct action meters and differential-action meters, the classification depending on whether in revolv ing the wheel the water does or does not exert a force which tends to retard the wheel's mo tion. In the direct-action type the wheel carries vanes (either flat or warped) set on a horizontal axis and revolving by the direct pressure of the moving water against the vanes. In this form of meter the friction increases as the velocity decreases. In the differential meter the wheel is made up of a series of cups at the extremities of short spokes. This wheel re volves on a vertical axis. The pressure of the current of water on the concave side of the cup is greater than on the convex side in the proportion of 100 to 38. The wheel revolves
slower than the direct-action wheel and die friction increases with the velocity, but is over come by the increased motive power developed by the velocity. The current meter is usually equipped with a tail which holds it steady in the current. It may be attached rigidly to rods from an overhead bridge or hung by a cable. In the latter case weights are attached below • to hold it in position. In high velocities or deep streams guy wires are attached. The recording mechanism is on the bridge or on shore, connection being made by an electric cable, and the record is made by electricity supplied by a battery. The true relation be tween the revolutions of the meter wheel and any velocity of a stream must be established individually for each meter before it is used.
This testing is called rating. A current meter is rated by drawing it through still water at a uniform speed and noting the number of revo lutions as to time and distance, from which the number of revolutions per second is computed. Many such experimental runs are made with each meter at different .speeds and the results are so recorded as to be quickly available in computing current velocities from any record that individual meter may make.