﻿772 Dr. A. Sellerio on the Repulsive Effect upon 



7. The next point to be considered, as I pointed out loc.cit., 



is the specific pressure p = rr; that is, the pressure upon the 



unit surface oil the crater, A being the whole area. 



Taking from Duffield's fig. 22 the values P (as unaffected 

 by E, V, and corrected for C), and, on the other hand, from 

 my fig. 10 the values A for a circular crater normal to the 

 carbon rod BM, we obtain 



Table II. 



I. 2. 4. 6. 8. 10. amp. 



Ca n tfode r ] P °' 10 044 °- 90 1>43 212 f^ ne 



Anode A O017 0088 016 0-24 031 cm. 2 



Cathode A 0'005 0"O27 0-050 073 0096 > > 



Auode^ 5-8 5"0 5-6 5'9 6"8 dyne/cm. 2 



Cathode^ 20 16"5 18 19"5 22 >> 



This table shows that the unit force does not vary much, 

 with the current strength. 



It must not be forgotten that, owing to the uncertainty in 

 the data P, A, the above values are only recorded in order 

 to give a rough estimate of the specific pressure p>- To 

 pursue accurately this inquiry, coherent values of P and A 

 for the same carbons and the same arc length are required. 



Besides the mechanical pressure, recent manometric obser- 

 vations with drilled carbons have shown for current strength 

 under 20 amp. * a hydrostatic pressure up to 30 dyne/cm. 2 , 

 an order of magnitude not far away from that of the 

 mechanical pressure p. 



In a research f on the electric arc Hg + /C~ between Hg as 

 anode and a thin carbon rod as cathode, I have calculated a 

 pressure of 6500 dyne/cm. 2 upon the positive pole Hg. In 

 fact, it is known that in a mercu?-y arc a cavity of 1 mm. or 

 more in depth has been often observed J, corresponding to a 

 pressure range over 1300 dyne/cm. 2 , with currents of a few 

 amperes. With a carbon-carbon arc the specific pressure^ 

 is, as shown, a hundredfold smaller. 



* H. E. G. Beer and A. M. Tyndall, Phil. Mag. xlii. p. 95G (1921). 



t A. Sellerio, " Contribute alio studio quantitativo dell' arco eletrico 

 fra mercurio e carbone," Nuovo Cim. xxiii. jan -febr. 1922. 



X Stark u. Reich. P/ujs. Zeit. iv. p. 323 (1902). See also, Stark u. 

 Cassuto, Phys. Zeit. v. p. 269 (1904). 



