ARC IN AIR BETWEEN CARBON ELECTRODES 51 



netic field and found that it was deflected alternately, as 

 if it was wire which was carrying current. 



In 1878 Wilde 1 showed that the reason an arc between 

 two parallel carbons travels away from the ends at which 

 the current enters the carbons is that the magnetic action 

 of the current upon itself forces it in this direction. 

 Somewhat recently Morris 2 examined, by means of an oscil- 

 lograph, the current through an arc just before it was 

 broken by being deflected by a magnet and showed that 

 it was a pulsating current. This accounts for the shriek 

 which the arc gives when it is thus extinguished. 3 



While making photometric measurements with rotating 

 sectors, Trotter 4 discovered that there is a bright spot 

 near the middle of the crater which rotates with frequen- 

 cies varying from 50 to 450 per second, the most common 

 being about 100 per second. This was most noticeable 

 with short humming arcs. The direction of rotation 

 varied without any known cause. 



Dewar 5 showed that the vapors of the arc exert a pres- 

 sure on the electrodes, that on the anode being the greater. 

 This has been verified by Cheneveau. 6 



1 Nature, 19, 152; 1878. 



2 Lond. Elec., 59, 707; 1907. 



3 Other experiments with an arc in a magnetic field have been performed 

 by the following: 



Cassellman, Pogg. Ann., 63, 589; 1844. 



De la Rive, Phil. Trans., part i, 37; 1847. 



Quet, C. R., 34, 805; 1821. 



Joubert, C. R., 91, 161; 1880. 



Martiny, Dissertation Rostock and Ber. d. Phys. Ges., 2, 199; 1904. 



Child, Phys. Rev., 20, 100; 1905, and 24, 498; 1907. 



4 Lond. Elec., 33, 297; 1894. 



6 Chem. News, 45, 37; 1882. Proc. Roy. Soc., 33, 262; 1882. 

 8 Eel. Elec., 20, 402; 1899. 



