on the Electric Arc. 129 



to ten diameters. Its length could be kept constant by 



the lc 

 Fig. 2. 



means of the screw holding the lower terminal. 



When carbon terminals were used, the ends were pre- 

 viously shaped into the general form they would after use 

 have assumed. By this means much time was saved, as it 

 appeared that, with a given arc-length and current, terminals 

 once shaped do not pive variable volt readings according to 

 the time the arc has burned, but may be considered in their 

 normal state as soon as they have reached their normal 

 temperature. 



When metal terminals were used, the ends were rounded, 

 but it was found better not to make them too pointed, o wing- 

 to the tendency of the arc to bow out and become of 

 uncertain and variable length. 



Description of Arcs. 

 In what follows, the arcs will be designated by the 

 chemical symbols of their electrodes, followed by the gas 

 in which they occur. Thus, Cu-C in H. The first elec- 

 trode is always positive, and, except where carbon, or other- 

 wise stated, is the upper terminal, cooled by water circulation. 

 Observations were taken on arcs in air and hydrogen, using 

 carbon, copper, iron, and aluminium in all combinations, both 

 positive and negative ; also several of them in coal-gas. 

 The following is a summary of the observed phenomena. 



Arcs in Air. 



Cu-G and Fe-C arcs are probably more or less familiar. 



Al-C The characteristic aluminium flame is pale blue, 

 being hardly distinguishable from that of iron. The Al 

 was oxidized rapidly, but was only slightly eaten away. 

 Very little deposit was left on the globe and framework. 

 A strong odour was emitted, however, but a match 

 applied at the end of the tube did not light the gas 

 which should have been escaping. The arc could be 

 lengthened to 0"5 in. with 10 amperes. 



Phil. Mag. S. 6. Vol. 14. No. 79. July 1907. K 



