1886.] On Fluted Craterless Carbons for Arc Lighting, 501 



E.M.F 



Mean current . . . 



Diameter of carbons (cylindrical) 

 „ crater in carbon. . . . 



Mean intensity of arc measured 



in the horizontal plane (candle 



units 



Light per square millimetre of 



carbon section (candle units) 



It will be observed from this statement that the intensity of the arc 

 in the horizontal plane per square millimetre of sectional area of 

 carbon is about 35 per cent, less than it was with the small square 

 carbons used by Holmes, although it might reasonably be expected 

 that, with the improvements since effected in the manufacture of 

 carbons, the efficiency of the old small carbons would at least be 

 maintained. The relative efficiency of the large carbous used with 

 the powerful currents now available appears to be due, 1st, to the loss 

 of a large portion of the most intense part of the arc which is confined 

 within the crater of each carbon ; and, 2nd, to the fluctuations in the 

 intensity of the arc caused by the current passing between various 

 parts of the end of each carbon. For a .new electric light installation, 

 about to be made by the Trinity House at St. Catherine's Lighthouse, 

 Isle of Wight, it is intended to utilise the large " De Meritens " 

 machines that were used at the recent South Foreland experiments 

 for determining the relative merits of electricity, gas, and oil as light- 

 house illuminants. The electric light at St. Catherine's is intended 

 to be " single flashing " at periods of 30 seconds. Each flash is to 

 have a duration of 5J seconds, followed by an eclipse of 24J seconds. 

 It is intended to use one " De Meritens " machine during clear 

 weather, and two whenever the atmosphere is found to be so impaired 

 for the transmission of light, that the flashes are not reaching their 

 advertised range. The' defect here arose, which is common to all 

 electric flashing lights where a minimum and a maximum intensity 

 are adopted, viz., that the duration of the flashes of minimum and 

 maximum intensity would vary in the ratio of the difference in the 

 diameters of the carbons employed with one and two machines 

 respectively, which in this case should be 50 mm. and 35 mm., this 

 mean difference amounting to 36J per cent, nearly. 



It is evident that such a variation in the duration of flash would 

 seriously impair the distinctive character of the signal. It occurred 

 to me, however, that, if carbons of a fluted section were employed for 

 the arc of minimum intensity whose extreme diameter corresponded 



Two machines 

 supplying currents 

 One machine. to one lamp. 



38 volts 48 volts. 



206 amperes 372 amperes. 



35 mm. 50 mm. 



13 „ 18 „ 



15,000 30,000 

 12 12 



