ELEMENTS OF ELECTRICAL ENGINEERING. 



An irregular grease spot on the thin paper screen enables one 

 to judge better when the illumination is the same on the two 

 sides. This spot should be made with clean paraffin and the 

 excess of paraffin should be drawn out of the screen by placing 

 it between folds of absorbent paper and applying a hot flat-iron. 



In the use of the Bunsen photometer a carbon-filament electric 

 lamp is generally used as a working standard. This lamp is 

 previously standardized for a particular voltage and in a particular 

 direction, by comparing it with a Hefner lamp ; and when used 

 it is operated at this particular voltage. Carefully standardized 

 carbon-filament lamps are offered for sale by the United States 

 Bureau of Standards, Washington, D. C. 



Fig. 1 75. 



130. Distribution of light around a lamp. In the definition of 

 the spherical-hefner the idea of uniformity of distribution of light 

 around the lamp was introduced for the sake of simplicity. In 

 fact, however, no lamp gives complete uniformity of distribution, 

 but the conical intensity in hefners or candles is always greater 

 in certain directions and less in other directions. Thus Fig. 175 * 



* Taken from a paper by J. R. Cravath and V. R. Lansingh on Reflectors, Shades 

 and Globes, Electrical World and Engineer, Vol. 46, pp. 907, 947, 991, 1033 and 

 1074, November 25 to December 23, 1905. 



