rays. This current is a measure of the heat falling upon the exposed band, and is easily converted from ampères into heat units.
Actinometers are similar in principle to pyrheliometers, except that while the readings of the latter may be directly converted into heat units by means of the known constants of the instruments, the constants of the former are unknown, and comparative readings with a pyrheliometer are necessary before their indications can be reduced to heat units.
Observations with pyrheliometers and actinometers are subject to errors from several different sources that will not be here discussed.
THOMPSON SILVANUS P., "Light, Visible and Invisible," New York, etc., 1897.
MAXWELL J. CLERK, "Theory of Heat," London, etc., 1902.
ÅNGSTRöM KNUT, "The Absolute Determination of the Radiation of Heat with the Electric Compensation Pyrheliometer," Astrophysical Journal, Chicago, v. 9, 1899, pp. 332-346.
SMITHSONIAN INSTITUTION, Annals of the Astrophysical Observatory, v. 2, Washington, 1908.