ACTINOM'ETER, an instrument for measuring the intensity of the chemical action of the sun's rays. For use in photography for the judging of times of exposure, the essential part of the instrument is a strip of sensitive silver paper which is blackened by the sun's rays, the time required to darken the paper tp a definite shade being taken as the index to the intensity of the light. Any other chemical action that light rays are capable of performing may be made the basis of an actinometer; but the indications of instruments in which the fundamental chemical changes are different will not necessarily agree with one another, be cause any given actinometer shows nothing but the intensity of the particular part of the spec trum which performs the chemical change upon which that instrument is based. Sir John Herschel's actinometer was a thermometer whose bulb contained a blue solution of sul phate of copper and ammonia. The expansion of this solution, by absorbing the sun's rays, measured the quantity of chemical rays in the sunshine. Bunsen, Draper and Roscoe selected instruments sensitive to certain selected radia tions, such as cause chemical or other changes.
In this case the sun's rays perform molecular work and are measured by the effects. For example, when a chlorine and hydrogen mix ture is converted into hydrochloric acid, the quantity of acid produced in a given period is a measure of the intensity of the rays which produced it. Ferric-oxalate and chloride of iron dissolved in water and exposed to the sun's rays give out carbonic acid gas and the quantity of the latter given off may be used as a basis of measuring the intensity of the rays. A photographic plate exposed for a short period of time receives an impression the intensity of which is determined by a scale of tints or shades, and this intensity of the i impression in a given period is made the basis for determining the intensity of the sunshine. When the sensitive element, used as a measur ing medium, absorbs all the radiation of all wave-lengths the instrument is more properly called a pyrheliometer (q.v.),