Experimental details of the capillary height method

The measurement of the height of the bottom of the meniscus has theoretically to be made relative to a plane surface of liquid in communication with the capillary tube. It is not always recognized that a very large surface is required, if it is to be sufficiently plane the cylindrical tubes of 20 mm. diameter often used have a decided capillary rise. Tubes of 40 mm. diameter or more must be used if the surface is to be considered as plane. Richards and Carver have, however, tested Rayleigh s formula (10) for wide tubes, and found it to agree with experiment within a few thousandths of a millimetre for a tube 38 mm. diameter this formula can be used with fair accuracy down to tubes of 25 mm. diameter, as a correction to the level in the wide tube. The actual measurement of the difference in height between the two levels requires an accurate cathetometer and suitable illumination of the menisci (see Richards and Coombs). 

An accurately circular cross-section is not necessary some workers have laid unnecessary stress on the attainment of this. The height of rise is proportional to the sum of the reciprocals of the principal radii of curvature of the meniscus, which will be nearly equal to the major and minor semi-axes of the tube, rx and r2, if the section is elliptical the error in the 

The contact angle between the glass and the liquid must be zero. If the glass is properly cleaned the contact angle is almost always zero with 

The contact angle may not, however, be zero if much evaporation takes place at the edge of the liquid, or if the liquid is advancing up the tube. The capillary height should be measured with a falling meniscus. 

The following results obtained by this method provide easily accessible liquids of known surface tension, which may be used as standards for calibrating some of the instruments which will be described. 

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