CANDLE-POWER OF LIGHTS.

Auckland Star, Volume LVI, Issue 184, 6 August 1925, Page 13

CANDLE-POWER OF LIGHTS.

i HOW IT IS MEASURED. , When we hear of an electric bulb . being of a certain candle-power, and , of a huge arc-lamp giving an illumina- ( tion equivalent to that of say, 20,000 j candles, we naturally ask ourselves how ( such measurement of light is arrived at. The reckoning of candle-power is based ' first of all, upon the standard "candle" as laid down in the British Acts of Par- . liament which deal with gas and electric lighting undertakings. This "candle" is : a uniform one, made from spermaceti or whale .oil, weighs six to the pound, and 1 burns at the even rate of 120 grains to ' the hour. , In measuring candle-power the test • is carried out by means of a thin sheet of special paper, in the centre of which [ is a greased or waxed spot. The paper ; is held upright in a frame, and on either side -of it is placed—on an exact level with the greased spot—the standard "candle" and the light of which the I candle T pow,er is being measured. . , Both lights are then moved backwards t and forwards until they occupy positions , when the greased spot is no longer r visible on either side of the paper. When I this occurs it is known that the lights , diffused on each side of the paper are of ; equal strength. The "candle" and the , light being tested, however, are now at different distances from the paper, the former being perhaps a foot away, and the latter four feet. It is obvious then - that the second light is four times as j strong as the "candle," but since it r illuminates sideways as well as lengthi ways, this area must be squared, and - the measurement of sixteen candle-power is thus obtained. ' As even the best candles are not t always reliable, candle-power measure- . ments are usually caried out with a , standard lamp which has a light equival- . ent to ten candle-power.