Instruments for Observing 
Current Pole and Log Line 
25. The simplest form of apparatus for observing currents consists of the current 
pole and log line. By means of this apparatus both velocity and direction of the 
surface currents may be obtained. The current pole in general use is about 3 inches 
in diameter and 15 feet long; a shorter pole is used for shoal water. The pole is weighted 
at the lower end to float upright with the top about one foot out of water. Sheet lead 
or an iron weight cast in rings is used for this purpose and the weight may be changed 
to adapt the pole for observations in waters of different densities. 
26. The log line, which is attached to the current pole, is about three-sixteenths 
inch in diameter and several hundred feet in length. It is carried on a reel that is 
mounted on a stand. The line is graduated in such a manner that the velocity of the 
current expressed in knots and tenths is indicated directly by the amount of line which 
is carried out by the current pole in a specified interval of time. The time interval 
generally adopted is 60 seconds, and since a current with a velocity of one knot (6,080 
feet per hour) will carry the log line a distance of 1/60 of 6,080 or 101.33 feet per minute, 
the unit graduations of the line are spaced at 101.33 feet. Subdivisions for tenths of 
a knot are spaced at 10.13 feet. 
27. The unit graduations are marked by pieces of cotton string in which knots 
are tied, the number of knots in any string indicating the velocity of the current ex- 
pressed in knots or nautical miles per hour. The initial point or zero of the graduations 
should be marked in a distinctive manner. The subdivisions for tenths of knots are 
marked by pieces of oiled string according to the following scheme: For each of the 
fractions 0.1, 0.2, 0.3, and 0.4, a single string with one, two, three, and four knots 
respectively; for the fraction 0.5, a double string without knots; and for each of the 
fractions 0.6, 0.7, 0.8, and 0.9, a double string with one, two, three, and four knots 
respectively. The line should be wet when graduated and when in use. 
28. The stray line is an ungraduated portion of line between the current pole and 
the initial graduation of the log line. The stray line should have a length of about 100 
feet to permit the pole to attain a position beyond the effect of the disturbed waters 
in the wake of the vessel before beginning to measure the velocity of the current. 
That portion of the stray line attached directly to the current pole and used to lift 
the latter out of the water should be somewhat larger and stronger than the log line itself. 
29. Observing with current pole and log line-—A stop watch is now generally 
used in fixing the duration of run for the current pole. With the pole in the water and 
everything in readiness at the appointed time for taking the observation, the stray 
line is paid out and as the zero or initial point of the graduated part of the line passes 
a fixed reference mark on the vessel, the stem of the stop watch is pressed. The paying 
out of the graduated line is then continued until 1 minute has elapsed. The line is 
then stopped and the velocity of the current in knots and tenths read directly from the 
graduation nearest the reference mark on the vessel and the reading entered in a record 
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