INSTRUMENTS FOR OBSERVING 35 
87. Rating table.—Results of calibrations of radio current meters in use by this 
Survey show sufficient uniformity to justify the use of a single rating table for all the 
radio current meters now on hand. From Bureau of Standards calibrations of eight 
of the instruments, a rating formula, representing the average of the eight calibrations, 
has been derived. The rating formula follows: 
Let V=velocity of current in knots and 
N=number of seconds between velocity contacts 
_3.9795 
Then V= WN +0.041 
The above formula was used in the construction of Table 5 on page 50. ‘This table 
may be used for any radio current meter now in use by this Bureau. 
Pettersson Current Meter 
88. The Pettersson current meter, originally designed by Prof. O. Pettersson and 
later modified by Dr. Hans Pettersson, has been used to a limited extent by this Bureau. 
It is complicated and weighs about 200 pounds. The meter is designed to give a photo- 
eraphic record of both the velocity and the direction of the current and will operate 
automatically without an attendant for about 2 weeks. The recording apparatus is 
contained in a watertight cylinder, below which is an anemometer wheel that is actu- 
ated by the current. The movement of the anemometer wheel is transmitted to the 
recording apparatus inside the cylinder by means of a magnetic drive. 
89. The cylinder contains a tiny camera with a roll of film that is moved by 
clockwork, a small electric lamp with batteries, a glass velocity disk with numerals 
inscribed near the outer edge, a glass compass disk carrying two magnetic needles and 
inscribed with numerals to indicate direction, and a system of reducing gears connecting 
with the velocity disk. At intervals of 30 minutes the electric lamp is automatically 
flashed and a picture is taken showing numerals on each of the two disks. The numeral 
on the compass disk indicates the direction of the current at the time the picture was 
taken, and the numeral on the velocity disk indicates the accumulated revolutions of 
this disk resulting from the rotation of the anemometer wheel. ‘The difference between 
two successive readings of the velocity disk, as interpreted by a rating table, will give 
the average velocity of the current for the half-hour period between the light flashes. 
A detailed description of this meter with instructions pertaining to its use is contained 
in Coast and Geodetic Survey Special Publication No. 124, Instructions for Tidal 
Current Surveys. A description by Dr. Hans Pettersson will be found in the Quarterly 
Journal of the Royal Meteorological Society (London), vol. XLI, No. 173, January 
1915. 
