THE IRRIGATION AGE 



239- 



therefore, to know, for the particular meter used, the 

 relation of its velocity of revolution to the velocity of 

 the stream's flow, or, in short, to know the meter's equa- 

 tion, or rating, as it is commonly termed. Before pro- 

 ceeding, however, to a description of the method of 

 rating the meter it will be well to describe with some 



EATING THE CURRENT METER. 



The equation of a meter is determined by attaching 

 the instrument to the prow of a rowboat, or to specially 

 constructed float or car and moving the vane through 

 still water at various rates of speed ; varying from three- 

 tenths feet per second to ten or twelve feet per second. 



Fig. 5. Flume Cross-Section, Showing Methods of Manipulating Current Meter. 



detail the mechanism of the meter itself. From what 

 has been said it will be seen that it is necessary by some 

 means to keep tally on the number of revolutions the vane 

 makes when immersed in flowing water. This is ac- 

 complished in a variety of ways, depending upon the 

 style or type of meter used. In Pig. 4 is shown a small 

 sized current meter, suitable for measurement of all 

 sized canals and for medium sized streams. The meter 

 here shown has the vane mounted on a vertical axis, 

 which at its upper extremity is made eccentric so that 

 during a portion of each revolution of the wheel the 

 eccentric portion of the shaft comes in contact with a 

 fine steel spring, and thus makes an electrical contact. 

 By means of an electrical buzzer or register, properly 

 connected in circuit with the steel spring and meter 

 frame, the revolutions of the vane may be counted by 



The observations are then platted on co-ordinate paper 

 with the velocity in feet per second as ordinates and the 

 revolutions of the vane per second as abscissae. For 

 such an instrument as the one illustrated, the curve 

 connecting the platted points will be practically a 

 straight line, whose equation may be easily determined. 

 By using the equation or by reading directly from the 

 curve, a rating table may be constructed .showing the 

 velocity corresponding to any number of revolutions 

 per second. The table is then ready for use in calcu- 

 lating the results of a stream measurement. 



USING THE CURRENT METER. 



To determine the discharge in any channel two 

 things must be determined : first, the sectional area of 

 the stream at the point of measurement, and second, the 

 velocity with which the water passes through different 



Section of 100 Foot Stream Showing Method of Division Into Small Sections. 



the observer. In the figure the electric buzzer is shown 

 connected to the two binding posts of the meter. There 

 are other forms of electric current meter, some having 

 horizontal axis and provided with a helicoidal shaped 

 vane instead of the cup vane. For large river work the 

 meter is made much heavier and stronger throughout, in 

 order that it may withstand the shock and strain without 

 being damaged. In addition to the electric pattern there 

 are what are 'termed acoustic current meters, in which 

 the revolutions of the vane are indicated by a mechanism 

 which taps on a small metallic drum every tenth revolu- 

 tion, the sound being transmitted through the meter 

 rod and a rubber tubing connection to a receiver placed 

 to the observer's ear. Then, also, there are self-recording 

 current meters in which the vane engages, by a system 

 of gears, with a set of index wheels from which the 

 revolutions mav be read directlv. 



portions of this cross-section. The former can be de- 

 termined by direct measurement, as will be pres- 

 ently described. The latter must be determined 

 by means of the current meter, or some other 

 means, such as floats. The velocity in any cross-section, 

 as we all know, varies widely at different points. In the 

 center of the stream the velocity is much greater than 

 along the banks or bed. To obtain, therefore, an ex- 

 pression which shall represent th.e mean velocity 

 throughout the section it is necessary that observations 

 be taken at various points. In practice the mean ve- 

 locity is not determined for the stream as a whole, but 

 the cross section is sub-divided into small sections in 

 which the velocity and discharge are determined inde- 

 pendently, the total flow of the stream being represented 

 by the sum of the discharges in the smaller sections. 

 In choosing a point of measurement on a canal, 



