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This method requires a rod with a platinum wire 1 mm. in diameter inserted in it about 1 inch from one end of the rod and projecting from it at a right angle at least 25 mm. Near the other end of the rod, at a distance of 1.2 meters from the platinum wire, a small ring shall be placed directly above the wire through which, with his eye directly above the ring, the observer shall look when making the examination.

The rod shall be graduated as follows: The graduation mark of 100 shall be placed on the rod at a distance of 100 mm. from the center of the wire. Other graduations shall be made according to Table 1, which is based on the best obtainable data. The distances recorded in Table 1 are intended to be such that when the water is diluted the turbidity readings will decrease in the same proportion as the percentage of the original water in the mixture. These graduations are those on what is known as the U. S. Geological Survey Turbidity Rod of 1902.^[105]

Table 1.—Graduation of turbidity rod. Turbidity (parts per million). Vanishing depth of wire (mm.). 7 1095 8 971 9 873 10 794 11 729 12 674 13 627 14 587 15 551 16 520 17 493 18 468 19 446 20 426 22 391 24 361 26 336 28 314 30 296 35 257 40 228 45 205 50 187 55 171 60 158 65 147 70 138 75 130 80 122 85 116 90 110 95 105 100 100 110 93 120 86 130 81 140 76 150 72 160 68.7 180 62.4 200 57.4 250 49.1 300 43.2 350 38.8 400 35.4 500 30.9 600 27.7 800 23.4 1000 20.9 1500 17.1 2000 14.8 3000 12.1

Procedure.—Lower the rod vertically into the water as far as the wire can be seen and read the level of the surface of the water on the graduated scale. This will indicate the turbidity.