INSTRUMENTS FOR OBSERVING 29 
to the main part of the instrument by right-handed unions, which are pulled tight with 
a key. 
77. When preparing to take an observation, the meter should be checked to see 
that the screw turns freely and that the bronze balls are deposited regularly according 
to the speed with which the screw is rotated. The receptacle below the compass box 
should be emptied of any balls it may contain and the magazine filled if necessary. The 
dial readings should be recorded or the pointers turned to zero. The dial pointers are 
adjustable, being held on their shafts by friction, but frequent adjustments may cause 
the pointers to become loose and drop off. Except at the beginning of the work, the 
pointers should not be changed. The practice of letting the last dial reading of each 
measurement become the initial reading for the following measurement affords some 
check against any large error that might be made in the reading and also is the most 
convenient method for doing the work. 
78. Before placing the meter in the water the two front shutters are closed and 
secured by the end of the locking lever. This also locks the screw so that it cannot 
turn until released by the first messenger. The twin-spring lever must be pressed 
towards the supporting cable and secured by the ratchet provided for the purpose. 
This is important and if not done, the screw will not be released when the messenger is 
dropped. 
79. After the meter has been lowered to the depth at which an observation is to 
be taken, one of the messengers is attached to the cable and allowed to slide down, the 
time of release being recorded. After the lapse of any specified time interval that has 
been selected for the purpose (a 10-minute interval is frequently used), a second mes- 
senger is dropped. The meter is then drawn to the surface and the dials read and 
recorded. The receptacle containing the deposited bronze balls is removed from the 
instrument and placed in the graduated reading frame. The direction of the current 
as indicated by the compartment containing the balls is recorded. Balls may be 
found in more than one compartment and in this case the average direction as indicated 
by the compartment containing a preponderance of the balls is taken. When it is 
desired to resume observations with a minimum delay, an extra receptacle may be used 
and the meter can then be returned to the water immediately after the dial reading has 
been taken and the instrument reset. 
80. Rating table.—Each meter must be calibrated before using in order to determine 
the relation of the velocity of the current to the speed of rotation of the meter wheel or 
screw. In the Ekman meter this relation will depend largely upon the shape and pitch 
of the blades of the screw. Each meter is usually provided with an extra screw which 
must be calibrated separately. The rating formulas given below were determined 
from Bureau of Standards calibrations of Ekman current meter No. 110, using screws 
Nos. 219 and 220. 
Let V=velocity of current in knots, and 
N=number of revolutions of the screw per minute. 
Then V=0.0110N-+0.020 for screw No. 219 and 
V=0.0116N-+0.020 for screw No. 220. 
The above formulas were used in the construction of Table 4 on page 50. Although 
this table is prepared for an observational period of 1 minute, it can be used with equal 
convenience if the observational period covers 10 minutes by simply entering the table 
with a number one-tenth as great as given by the meter. In general it will be better 
to use the 10-minute time interval for this meter. 
