stone bridge may, therefore, be considered as made up of ohmic resistance, and only a single balance for resistance, as in a direct current bridge, is necessary.

The heating coefficient, i.e., temperature rise per watt dissipated in the cell, for a stirred bath was found to be only 0.1°C. and did not change appreciably with the rate of stirring. In an unstirred bath the coefficient was about 0.3°C. For operating the recorder about 0.05 ampere in the cell is needed. This will raise the temperature of the sealed cell about one-fortieth of a degree above the temperature of the bath and open cell.

The differential temperature coefficient which is here expressed as the per cent change in the ratio of the resistances of the two cells, containing different solutions, per degree change in the temperature of the bath in which the two cells are immersed was found at various temperatures.

The following values of this coefficient were obtained for salinities of 29 and 32 grams per 1000 in the two cells respectively.

from 0 to 10°-0.00013

10 to 20°-0.00009

20 to 30°-0.00007

From these values a small correction can be applied if the temperature of the sea water is different from the temperature at which the calibration of the instrument is made.

The thermal time constant of the cells, which is the time necessary for the temperature of the cell to approach the temperature of the bath to 67 per cent of its initial difference in temperature, was found to be small. For a stirred bath the time constant of the sealed cell is 37 seconds and the open cell 34 seconds, and as in the case of the heating coefficient these values did not change much with the rate of stirring. For an unstirred bath the time constant was 62 seconds. As stated in the previous paper this small time constant is sufficient to bring the cells quickly to the temperature of the bath for the maximum change occurring in the temperature of the sea water.

The effect of flow in the open cell was determined by passing a thoroughly mixed solution through the cell from a large sup

ply. No change in conductivity was obtained up to a flow of as high as 1 liter per minute.

The efficiency of washing out the cell was obtained by use of two solutions, one having a salinity of 35 and the other a salinity of 32. These solutions were passed through the cell simultaneously and the resistance of the cell was measured at definite intervals. For a slow flow (100 cc. per minute) the cell washed out to within 0.01 of the salinity of the second solution, after 400 cc. of this solution had been used. For a rapid flow this degree of washing-out was attained with only 300 cc. of the solution. This is much better than was obtained in the preliminary experiments on other cells; and is sufficiently rapid for the maximum change in salinity which occurs in the ocean.

A few experiments to find the effect of air bubbles showed that bubbles on the back of the electrodes caused no change in conductivity and bubbles up to 3 mm. in diameter just in front of the electrodes gave no change in conductivity. A bubble 2 mm. in diameter in one of the tubes gave a change in conductivity corresponding to 0.01 in salinity. By tilting the cells 15 degrees any bubbles which might form in the cells will come to the surface above the electrodes.

Fig. 2. Resistance thermometer, electrolytic cells and bath. Sea water enters through the pipe (A) and passes by the resistance thermometer which is contained in the large pipe (B). The sea water overflows from this pipe and passes through the bath (C) and open cell (D) and then empties into the bilge of the vessel.

ELECTRICAL CONDUCTIVITY OF SEA WATER

The electrical conductivity of sea water was measured in the sealed cell placed in a stirred bath. The temperature was maintained at 25°C. to within 0.01°C. The resistance capacity of the sealed cell was measured with 1/10 N solutions of potassium and sodium chloride.2

The electrical measurements were made by substituting an accurate resistance box in place of the cell in one arm of a Wheatstone bridge circuit. The 60 cycle power circuit was used and no auxiliary capacity or inductance was used to compensate for the capacity of the cell. The resistance capacity was determined with one sodium chloride solution and two potassium chloride solutions. One of the potassium chloride solutions was specially prepared by the chemical division of the Bureau of Standards and the resistance capacity given by this solution has been used to measure the conductivity of sea water. Following are the capacities obtained with the three solutions at 18°C. using a frequency of 60 cycles:

1/10N, KCl prepared by the Bureau of Standards..
1/10N, KCl prepared from Kahlbaum's pure salt.
1/10N, pure NaCl..

5.182

5.181

5.178

Unfortunately the specific conductivity of standard solutions, at a frequency of 60 cycles, has not been measured. The resistance capacities given above are therefore only apparent and can be used to measure the specific conductivity of sea water provided the change in resistance with frequency is the same for a 1/10 normal solution of KCl as it is for sea water which has a strength of about 1/5 normal. The resistance capacities obtained with 1/10 normal KCl and 1/10 normal NaCl solutions indicate that the change in resistance with frequency is the same.

2 The specific conductivity of these solutions at 18°C. measured by F. Kohlrausch and M. E. Maltby in 1900 are 0.011203 and 0.009202 reciprocal ohms respectively. As prepared by Kohlrausch the 1/10 N solution of KCl used in these measurements contained 7.445 grams of KCl to one liter of solution at 18°C. and the 1/10 N solution of NaCl contained 5.848 grams of NaCl to one liter of solution at 18°C. All weighings were made in air and low conductivity distilled water was used.

for different solutions of the same concentration. Previous tests on the two cells in a bridge circuit also indicate that for small differences in concentration (salinity of 35 to salinity of 32) there was no appreciable change in resistance with frequency. The sea-water conductivity measurements have therefore been made with the assumption that the resistance change with frequency is the same for 1/10 N, KCl as it is for sea water.

The sea water for conductivity measurements was collected by the U.S. S. Androscoggin on April 9, 1918, in Latitude 30°-4'N and Longitude 67°-10'W. This water had a high salinity and the more dilute samples were prepared from it by adding distilled water. The one sample of a higher salinity was prepared by carefully distilling sea water. The distillate was measured for conductivity and the low conductivity obtained showed that no chemicals in the sea water were given off.