Reduction of Current Records 
122. On receipt of the field records in the office, it is necessary to check and inter- 
pret the data. The forms most commonly used for recording the original observational 
data are Form 270 Record of Current Observations and Form 270a Hourly Current 
Observations. The first is for general use and provides for both pole and meter obser- 
vations. The latter was designed especially for hourly pole observations at lightships. 
When an automatic recording device is used with the Price current meter, the record 
consists of a series of graphs on circular charts. Records from the radio current meter 
consist of chronograph-tape tracings together with the field interpretation of those 
tracings tabulated in copies of Form 270. Current records supplied by other organi- 
zations may be in various manuscript forms. 
Record of Current Velocity 
123. In the work of this Bureau, velocity of the current is usually expressed in 
knots, or nautical miles per hour, and this unit is generally indicated in the forms used 
in connection with the work. If velocities have been expressed in any other unit, care 
must be taken to have all papers marked accordingly. For convenience, reduction 
processes may be carried on with the original units used for the observations, but the 
final current velocities should be expressed in knots. The factor 0.592 applied to 
velocities given in feet per second, or the factor 0.0194 applied to velocities in centi- 
meters per second, will convert these velocities to knots. 
124. When observations have been taken with a current pole and line, the latter 
being suitably graduated, the velocities in knots and decimals are entered by the 
observer directly in the record book (Form 270 or 270a). When a Price current meter 
is used with earphone, the observer enters directly in Form 270 the number of revolu- 
tions of the meter wheel in a specified interval of time. The corresponding velocities 
are afterwards obtained by means of rating table (Table 2) and entered in the column 
provided for the purpose in the same form. ‘The rating table is directly applicable if 
the standard observational interval of 60 seconds has been used for the observations, 
but may be adapted to another observational interval by the use of an appropriate 
factor. For example, if an observational interval of 30 seconds has been used, the 
number of revolutions should be doubled before entering the table. 
125. For the interpretation of the graphs from the recording device sometimes 
used with the Price current meter, the following procedure involving the use of Table 
3 is recommended when the contact ratio is either 1:10 or 1:50. Prepare a curved scale 
covering a little more than 1 hour of the graphic record and divide it into 12-minute 
(0.2 hr.) spaces. In the middle of the first space make a distinctive mark which will 
be applied to the beginning of each hour in the graphic record. The first 12-minute 
space will then cover an interval from 6 minutes before the hour to 6 minutes after the 
hour and the average velocity for the period may be attributed to the exact hour. Also, 
the successive 12-minute periods may be taken as representative of the exact fractional 
_parts 0.2, 0.4, 0.6, and 0.8 of the hour. 
: 47 
