CURRENT-METER. A device for measur ing the velocity of sub-.surface currents, usually for the purpose of ascertaining the discharge of a stream or channel. (See blvnuounArtiv.) Cur rent-meters are made in several patterns, hut they are all of the same general form. A hori zontal metal frame or body carries at one end a vane-like tail, and at, the other end a bladed wheel free to rotate, the whole being attached at its centre of gravity to a vertical metal shaft so as to have a free, rocking motion between certain limits in the vertical plane. When sunk in flowing water by attaching, a suspending, wire or cord to the top of the shaft and an anchor weight to the bottom. so as to hold the shaft vertical and the meter-frame horizontal, the current acting on the vane-like tail. like the, wind on a weathercock, swings the instrument into coincidence with the direction of the cur rent, with the bladed wheel pointing against the current. The current, striking the blades of the wheel, causes the wheel to rotate at a speed varying with the velocity of the current, and a record of the speed of rotation is kept by means of an electrical circuit which is completed and broken by the wheel one more times each revolution. The recording apparatus is kept on shore or in a boat, while the meter is sus pended by suitable appliances at any point of the stream at which it is desired to measure the velocity of the current. With one exception, which is noticed further on, the chief difference between the different patterns of current-meter now in use exists in the rotating wheel. In the earliest form of meter, invented by Gen. Theo dore G. Ellis, the wheel has helicoidal blades, but in later forms conical cup-shaped vanes are employed. The invention of the electrical re cording attachment as applied to current-meters is credited to D. Ferrand Henry. of Detroit,
Mich. The Price current-meter shown in the illustration has an electrical recording device, and the wheels shown at A are of sueh shape that they feel the influence of a very slight cur rent. The weight which serves to keep the appa ratus submerged is shown at B. The most recent pattern of current-meter is the so called direction cu•rent-meter invented by E. S. Ritchie and E. E. Haskell, the latter of the United States Coast and Geodetic Survey. With this meter the observer is able to determine simultaneously on recording dials the direction and velocity of any current. This meter consists of a fish-like body, or chamber, mounted on horizontal bearing; carried by a ring which encircles the body. To the top of this ring is attached an eye for the suspension-cable con nection, and to the bottom is attached a similar eye from which the anchor weight is hung. The rear end of the body• is prolonged in the shape of a tail-vane, cruciform in section, and the for ward end terminates in a hollow shaft on to the end of which the wheel-hub is journaled. The wheel is of the screw-propeller type and conical In form, and its rotation is recorded by indica tors operated by an electrical circuit. The body of the meter is a compass, whose needle is free to assume the magnetic meridian, and by means of an electric circuit the angle between the direc tion in which the compass-needle points and the direction in which the axis of the instrument points is recorded on a dial. This meter is suf ficiently delicate to record a variation in velocity of current as small as 0.2 foot per second.