May as, 1914 
Cup Current Meters 
81 
indicated by the original curves, which followed the dotted lines of fig¬ 
ure 3, is 44.82 second feet, which is 2.14 per cent less than the discharge 
computed with proper corrections-applied. The mean velocity indicated 
by the solid curves is 1.76 feet per second. As influenced by the broken 
portions, it is 1.72 feet per second. For a given depth and a given mean 
velocity the difference in results with and without the proper corrections 
will vary approximately as the width of the channel, but for a given 
Fig. 4.—Curves on logarithmic scale, showing relation between rate of vertical movement of a cup meter 
and velocity of water as indicated by the revolutions of the turbine due to vertical movement in still 
water. 
width of channel the difference approximates a constant quantity with¬ 
out regard to depths and velocities rather than a percentage difference. 
The main point the writer desires to bring out is that a cup current 
meter should be actually rated at the surface and bottom of the channel 
if it is to be operated at these points in making discharge measurements. 
From the various rating tables thus developed the proper interpretation 
of the speed of the meter can be recorded. 
Runs with the meter held close against the vertical concrete wall of the 
channel indicate that the meter runs true to the standard curve, but dis¬ 
regards the cycloidal motion of the water. As the rating car passed to and 
fro over the channel, the meter was held close to the same wall, thus bring¬ 
ing the open and closed sides 1 of the turbine, alternately, next to the wall. 
