CURRENT METERS 357 



the majority of cases this is found to be of the form shown in Fig. 158. 

 At very low speeds the friction of the instrument varies between fairly 

 wide limits, but diminishes as the speed increases. This causes the 

 plotted points to lie more or less on a curved line, but renders this 

 portion of the calibration unreliable. At a certain critical speed of the 

 instrument the friction takes a fairly constant value, and the curve 

 becomes very approximately a straight line of the form 



v = a n -\- b. 



It is not advisable to use the meter to register speeds below the critical. 

 This depends on the instrument, but is usually from 3 to 6 inches per 

 second, and in general it may be taken that on this account the meter is 

 not a suitable instrument for the measurement of the discharge of a 

 stream if the velocity over more than 15 per cent, of its area is less than 

 6 inches per second. 



A form of current meter which is occasionally used consists of a flat 

 circular plate which is rigidly attached by means of a horizontal arm to 

 one end of a vertical wire, the other end of which is fixed. The wire is 

 supported in bearings, and the free end carries a pointer which, working 

 over a graduated disc, enables the angle of twist to be ascertained. 

 In using the instrument, the pointer is adjusted to zero with the 

 plate out of water and normal to the direction of flow of the stream. The 

 plate is then submerged and the angle of twist necessary to bring it once 

 more normal to the direction of flow is noted. 



Then if P = force on plate in Ibs. 



I = length of arm from centre of plate to centre of wire. 

 A area of plate in square feet. 

 6 = angle of twist of wire. 

 v = velocity of flow of stream. 



We have P I = K 0, where K is a constant for any instrument and 

 depends solely on the material, length, and radius of the wire. 



Also P = 1-15 A r 2 (approx.) the constant depending on the size of the 



plate, from which r 2 = - . 0. 



1*15 A I 



Even with a constant velocity of flow, however, eddy formation at the 

 rear of the plate causes the value of P to undergo periodic fluctuations, 

 and the difficulty in obtaining a true mean value for #, and in keeping 

 the plate normal to the direction of the stream, prevent this method from 

 having any pretensions to great accuracy. The instrument is now 

 practically obsolete. 



