ARC IN AIR BETWEEN CARBON ELECTRODES 9 



sure of 1500 to 1800 atmospheres. These rods are baked 

 until all volatile substances are driven off and are then cut 

 into the desired length. 



If the rods thus formed are homogeneous they are called 

 solid carbons. Cored carbons consist of an outer cylinder 

 which is made by the same process as that just described 

 and a core which is filled in after the outer cylinder is fin- 

 ished. According to Mahlke * the core consists of two parts 

 of baked lampblack and one part of potassium silicate. 

 The core usually has a diameter about one-fifth that of 

 the whole carbon. 



Relation Between Potential Difference, Current and 

 Length of Arc. One of the first questions for us to con- 

 sider is that concerning the amount of E.M.F. necessary 

 in order to maintain the arc. The first accurate measure- 

 ments taken for the purpose of answering this question 

 were made by Edlund in 1867. 2 He found that r = m -\- nl, 

 where r is the "apparent resistance," that is the ratio 

 between the voltage at the terminals of the arc and the 

 current; / its length; and m and n quantities which are con- 

 stants as long as the current is constant. He further stated 

 that the voltage of the arc is independent of the current, 

 and that the "true" resistance, represented by the term nl, 

 is proportional to the length and increases as the current 

 decreases. 



There were at that time no definite units for measuring 

 electrical quantities and no dynamo for producing the 

 current, so that the accuracy of Edlund's results seems to 

 have been the result of good fortune as well as of careful 



1 Elec. World., 57, 672; 1911. 



2 Pogg. Ann., 131, 586; 1867. 133, 353; 1868. 134, 250 and 337; 

 1868. 139, 354; 1870. Wied. Ann., 26, 518; 1885. 



