May 35, 1914 
Cup Current Meters 
79 
For the experiments made with a dulled pivot the pivot was roughly 
rounded on a carborundum wheel to a curve of about the same radius as 
the shaft—far duller than probably would ever be used in actual practice, 
as the wheel developed facets. The experiments with the very dull 
pivot show that for velocities of water greater than 1 foot per second 
the sharpness of the pivot is immaterial, the points determined by the 
experiments being closer to the standard curve than most of those from 
which this curve was developed. As might be expected, the added fric¬ 
tion due to the rounding of the pivot has more influence in low velocities, 
and the meter is a little slow under these conditions. 1 
On November 29 and December 1, 1913, the writer made experiments 
in the concrete channel of the hydraulic laboratory of Cornell University, 
with a new meter of the same type as that used in the canal measurements 
during the summer and in the Calexico experiments. The results of these 
runs are plotted to logarithmic scale in figure 2. 
These experiments indicate that for depths below the surface of 0.3 
foot or greater the meter runs true to the standard-rating curve at least 
within the range of velocities covered, which did not exceed 6 feet per 
second. When the center of the meter is less than 0.3 foot from the sur¬ 
face, the points indicate a peculiar behavior. With the center 0.2 foot 
below the surface the meter velocities followed the standard curve until 
a velocity of about 1.5 feet per second was reached. At this velocity the 
surface of the water above the meter starts to break up and the rating 
curve leaves the standard curve, as shown by the dash-and-dot line in 
figure 2, but it did not intersect the standard curve again within the range 
of the experiments. 
With the center of the meter submerged just 0.1 foot the curve leaves 
the standard curve at a lower velocity—about 1.15 feet per second— 
as is to be expected. The influence appears to reach a maximum at a 
velocity of about 2.2 feet per second, at which point the actual water 
velocity is about 0.45 foot per second higher than the revolutions of the 
meter would indicate on the basis of the standard curve. As the velocity 
increases, the two curves come closer together until the curve for the o. 1 
foot depth appears to cross the standard curve at a velocity of about 5.5 
feet per second. The writer can account for this strange action only on 
the theory that by the time this velocity is reached the entire surface of 
the water in the vicinity of the meter has become shattered, and enough 
wind pressure operates against the cups to turn the meter even faster than 
would be the case when submerged 1 foot. If a cup meter is held in the 
air, the great influence of a very slight wind is at once manifest. 
1 In correspondence, Mr. V. M. Cone, Irrigation Engineer, Office of Experiment Stations, points out “ that 
the ratings did not change materially on those pointed bearings if they were merely dulled, but the big 
trouble came when using the meter in comparatively swift water. There was a tendency for the turbine 
to lift, throwing the conical bearing upward, and very often a shoulder would be cut on the sloping face. 
Whether this is due to a mechanical defect in the instrument or to grit getting into the bearing I do not 
know, but wherever that groove or shoulder was cut the rating was very materially changed." 
