on 



the Electric Arc. 141 



For copper-copper in air, both solid rods 037 in. diam., 

 the positive above, 



Tr 1Q ,--' , 52-1 + 3-43/ 

 Y=18 + 17-5ZH -r . 



For copper-carbon in hydrogen, 



4-3 + 8G-4Z 



Y = 21-5 + 3-46Z + 



A 



For aluminium-carbon in hydrogen, 



Alternating- Current Ares. 



The carbon-carbon arc in hydrogen at about 50 volts, and 

 15 amperes at a frequency of 80, was steady to a length of 

 0'03 in. On first making contact, the carbons being cold, 

 the arc gave a momentary flash which was repeated at each 

 contact. By maintaining a bad contact for a few seconds, 

 until the carbons began to glow, and then slowly separating 

 them, the arc could be obtained, and was quite steady. It 

 could not be drawn out longer than 0*03 in. with 100 volts- 

 supply. ^ 



Copper-carbon in hydrogen gave no arc at 100 volts, 

 15 amperes and 80 cycles, nor again at 100 volts, 20 amperes 

 and 50 cycles. The arc apparently started at contact, but 

 went out at the first zero of the current after separation of 

 the electrodes. 



Carbon-carbon in coal-gas gave an arc of 0*01 in. and 

 0*02 in., the volts and amperes being about 38 and 14 

 respectively, and the frequency 80. At times the arc seemed 

 to reach 0*03 in. in length, but it did not appear to draw out 

 as long as the same arc in hydrogen. As in hydrogen, the 

 arc required a certain amount of previous heating of the 

 carbons to start. 



Copper-carbon in coal-gas gave at first no arc with 

 120 volts supply, and current of 15 amperes at 80 cycles. 

 Sparking and heating, however, gradually produced a de- 

 posit of carbon on the copper, and when this was sufficiently 

 thick, it was possible to maintain an arc of exceedingly short 

 length. 



Copper-carbon in air. The arc would not start until the 

 carbon had been heated to a glow. By this time a deposit 

 of carbon was on the copper. The arc could then be drawn 

 out to 005 in., but with some difficulty and with not less. 



