Electric Arc ivith Varying Power- Supply. 217 



Electrical Laboratory o£ University College, London, by 

 Prof. J. A. Fleming. 



The objects in view may be thus stated : — - 



(i.) To obtain a series of curves for both continuous and 

 alternating current arcs, showing the relation between 

 the mean spherical candle-power and the watts taken 

 up in the arc. 



(ii.) To compare the " efficiencies " of the continuous and 

 alternating arcs as illuminants when supplied at 

 different powers, the " efficiency " being expressed in 

 mean spherical candles per watt. 



As the field covered by these statements was too broad to 

 be covered in its entirety in the time at the author's disposal, 

 it was decided to impose the following restrictions : — 



The make and diameters of the carbons used to remain 

 unaltered throughout the experiments. 



Alternating current of one frequency only to be employed, 

 this frequency to be 80^ 



In continuous-current experiments the top carbon to be in- 

 variably positive. 



Arc-lengths not to exceed 0*5 inch. 



Power to be supplied at rates not greater than 1500 watts. 



The carbons used may be specified thus : — 

 Upper Carbon — , 



Maker C. Conradty, Nuernberg. 



Description Cored. Marke W.C. 



Diameter 12 mm. 



Diameter of core , 2 „ 



Weight per cub. centim 1*41 gramme, 



Weight per c.c. of outer carbon 1*43 „ 



Weight per c.c. of core 0'93 „ 



Lower Carbon — 



Maker C. Conradty, Nuernberg. 



Description Solid. Marke C. 



Diameter 10 mm. 



Weight per cub. centim. 1*46 gramme. 



The mean spherical candle-power was measured directly 

 by means of an integrating photometer. 



This instrument was described in detail in the paper referred 

 to above, and it will be sufficient to say that it consists of a 

 system of pairs of mirrors arranged in a semi-circle, about 

 6 feet in diameter, in such a manner that light emitted from 

 a source placed at the centre of the system shall be incident 

 upon a photometer-screen, also situated at the centre, at the 

 same angle with the vertical as that which it made on 

 emission from the source. 



