ARC IN AIR BETWEEN CARBON ELECTRODES 49 



3.1 mm. and with 20 amperes 6.4 mm. She also found 

 the diameter to increase slightly as the length of the arc 

 was increased. 



Granqvist 1 gives the following formula for the crater with 

 solid carbons, r = 0.043 + 0.008 /, where r is the radius in 

 centimeters and 7 is the current in amperes. As will be 

 noticed, these results are very different from those given 

 by Mrs. Ayrton for cored carbons. 



Reich 2 found that the bright spot on the cathode, which 

 he calls the negative crater, was larger, the shorter the arc. 

 With an arc 9 mm. long the following formula applied: 

 r = o.oi + 0.009 I centimeters. 



Enclosed Arc. With the common arc in air each of 

 the carbons is consumed at the rate of about one inch per 

 hour. This loss is principally due to oxidation of the 

 carbon. The simplest means of obviating this is to place 

 the arc in an air-tight globe, but it has been found that if 

 it is completely air-tight, carbon vapors and hydrocarbon 

 compounds diffuse to the side of the globe making it more 

 or less opaque. It was found, however, by Marks 3 that if 

 the arc is one with small current and sufficient length, so 

 as to require a comparatively high voltage, it is possible 

 to admit air in such small amounts that the rate of com- 

 bustion of the carbons is greatly reduced, while there is 

 still enough air to oxidize the carbon vapors. 



With the lamp first made by Marks there was a valve 

 which allowed a small amount of air to enter. With im- 

 proved carbons this has not been found necessary. There 

 is usually some chance for the leakage of air in the space 



1 Phys. ZS., 7, 79; 1906. 



2 Phys. ZS., 7, 73; 1906. 



3 Lond. Elec., 31, 502; 1893, and 38, 615; 1897. 



