Page:Handbook of Meteorology.djvu/268

256 to lift 33,000 pounds 1 foot high in a minute of time, is rated about 746 watts.

A coulomb is the unit of quantity. It is the quantity of electricity transferred by a current of 1 ampere in one second.

The farad is the unit of capacity. The capacity of a condenser is theoretically the electricity that can be stored in it by a cell of known electromotive force. For purposes of exact measurement the cell should have an electromotive force equal to 1 unit absolute measure. It is practically a condenser which, charged with 1 coulomb of current, has a difference of potential of 1 volt.

Freezing Mixtures.—Salt, 1 part; snow at 32°, 2 parts, produce a zero mixture.

Calcium chloride, 2 parts; snow at 32°, 1 part, produce a temperature of &minus;40° F.

Potassium hydrate (concentrated) at 32° and snow at 32°, equal parts, produce a temperature of &minus;30° F.

Ammonium chloride, potassium nitrate and water at 32°, equal parts, produce a temperature of &minus;10° F.

The temperature produced by these mixtures is approximate only. The first mentioned may vary not more than 3 degrees; usually the variation is not more than 1 degree; a variation of as much as 10 degrees may occur in the second. As a rule, the larger the volume of the mixture the better the result.

Chemical Hygroscopes.—Various chemical salts are sensitive to the moisture of the air; some of them change color with the absorption or the discharge of moisture. Cobaltic chloride is the basis of most of the commercial hygroscopes of this character. A solution consisting of 5 grains of cobaltic chloride, 50 grains of gelatine and 1 fluid ounce of water is a good porportion. There must be a complete emulsification of the gelatine. A strip of unsized paper wet with the emulsion is normally pink in moist air, violet in moderately moist air, and blue in dry air. When the winter fires are on in dwellings, the strip of paper is persistently blue. Ordinarily it does not begin to turn until the moisture of the air is about 75 per cent.

Ozone Test Papers.—Qualitative tests for ozone are often desirable. The following solution is highly regarded. Distilled