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4 TECHNIQUES OF WATER-RESOURCES INVESTIGATIONS the section extends laterally from one observa- tion point to the next. Discharge is the product of the average of two mean velocities, the average of two deptbs, and the distance between locations. A study by Young (1950) concluded that the midsection method is simpler to compute and is a slightly more accurate pro- cedure than the mean-section method. Current-meter measurements usually are classified in terms of the means used to cross the stream during the measurement, such as wading, cableway, bridge, boat, or ice. Instruments and equipment Current meters, timers, and' counting equip- ment are used when making conventional types of measurements. Additional equipment used depends on the type of measurements being made. Instruments and equipment used in making current-meter measurements are described in this section under the following categories: current meters, sounding equipment, width-measuring equipment, equipment as- semblies, and miscellaneous equipment. Current meters A current meter is an instrument used to measure the velocity of flowing water. The principle of operation is based on the pro- portionality between the velocity of the water and the resulting angular velocity of the meter rotor. By placing a current meter at a point in a stream and counting the number of revolu- tions of the rotor during a measured interval of time, the velocity of water at that point is determined. The number of revolutions of the rotor is obtained by an electrical circuit through the contact chamber. Contact points in the chamber are designed to complete an electrical circuit at selected frequencies of revolution. Contact chambers can be selected having contact points that will complete the circuit twice per revolution, once per revolution, or once per five revolutions. The electrical impulse produces an audible click in a headphone or registers a uuit on a counting device. The counting intervals are measured by a stopwatch. Current meters generally can be classified into two main types, those meters having vertical-axis rotors and those having horizontal- axis rotors. The comparative characteristics of these two types are summarized below: 1. Vertical-axis rotor with cups or vanes. a. Operates in lower velocities than do horizontal-axis meters. b. Bearings are well-protected from silty water. c. Rotor is repairable in the field without adversely affecting the rating. d. Single rotor serves for the entire range of velocities. 2. Horizontal-axis rotor with vanes. a. Rotor disturbs flow less than do verti- cal-axis rotors because of axial sym- metry with flow direction. b. Rotor is less likely to be entangled by debris than are vertical-axis rotors. c. Bearing friction is less than for vertical- axis rotors because bending mo- ments on the rotor are eliminated. ( Vertical-axis current meters The most common type of vertical-axis current meter is the Price meter, type AA. (See fig. 3.) This meter is used extensively by the Geological Survey. The standard Price meter has a rotor 5 inches in diameter and 2 inches high with six cone-shaped cups mounted on a stainless-steel shaft. A pivot bearing supports the rotor shaft. The contact chamber houses the upper part of the shaft and an an eccentric contact that wipes a bead of solder on a slender bronze wire (cat's whisker) attached to the binding post. A separate reduction gear (pentagear), wire, and binding post provide a contact each time the rotor makes five revolutions. A tailpiece keeps the meter pointing into the current. In addition to the standard type AA meter for general use there is a type AA meter for low velocities. No pentagear is provided. This modification reduces friction. The shaft usually has two eccentrics making two contacts per revolution. The low-velocity meter nor- mally is rated from 0.2 to 2.5 fps (feet per second) and is recommended when the mean velocity at a cross section is less than 1 fps. In addition to the type AA meters, the Geological Survey uses a Price pygmy meter