476 



Mr. J. E. Petavel. An Experimental 



3. Variations in the size of the carbons, in the intensity and density 

 of the current, in the length of the arc, and in the total power ex- 

 pended (as long as the arc is kept silent), will not cause the intrinsic 

 brilliancy to vary more than 10 per cent, on either side of the mean. 



4. No sensible variation in the intrinsic brilliancy, and therefore in 

 the temperature of the crater, is produced by placing the carbons in an 

 enclosure maintained at over 900° C. 



With regard to the constancy of the temperature of the crater, these 

 results are not without importance. 



Having this question in view, it was necessary to determine what 

 were the effects of extreme variations of current density and power. 



In Tables III and IV will be found the results of observations taken 

 when the arc was hissing.* 



Table III.— " Hissing " Arcs. Enclosure at 20° C. 



Electro- 

 motive 



force in 

 volts. 



Current 

 in 



amperes. 



Power in 

 watts. 



Diameter 

 of the 



positive 

 carbon 

 in mm. 



Current 

 density in 

 amperes 

 per sq. mm. 

 of cross - 

 sectional 

 area of the 

 positive 

 carbon. 



Length 

 of arc 

 in mm. 



Intrinsic 

 brilliancy 



of the 

 crater in 

 candle- 

 power 

 per 

 sq. mm. 1 



70 -0 



15 -8 



1106 



8 



0-314 



4-8 



136 



42 -0 



216 



907 



8 



0-410 



1-2 



i 143 



53-0 



25*8 



1367 



8 



0-514 



2-9 



157 



44 -0 



50-0 



2200 



15 



0-283 





160 



1 Each of the figures in this column is the mean of a number of photometric 

 observations. 



It will be seen that the current varied from 6 to 50 amperes, the 

 current density from 0*03 to 0*51 ampere per square millimetre, and 

 the power from 370 to 2800 watts. 



The lowest photometric readings gave 119, and the highest 160 

 candle power per square millimetre. 



* The word " hissing " is used here as being the generally accepted term. It is 

 only when the current density is small that it is actually descriptive of the sound 

 made ; as the current increases, the pitch rises, until with a very short arc and a 

 current density of about 1 ampere per square millimetre, the sound is between a 

 whistle and a scream. The arc then assumes a very peculiar aspect, a pointed blue 

 flame, like the flame of a blow-pipe, being sent out from the crater. This effect was 

 most marked when the high temperature eiiclosure was used. With the above 

 current density the entire carbon becomes white hot and burns away with great 

 rapidity ; an increase "n the intrinsic brilliancy seems also to take place. Unfortu- 

 nately, it was not found possible under these circumstances to obtain reliable photo- 

 metric observations. 



