402 Cady and Arnold — Electric Arc. 



For other metals, the data are too meager to afford comparison 

 between any except carbon and silver, and lead and silver. 

 With each of these as anode, the cathode consisted of carbon. 

 Among the observations recorded, the following are suitable 

 for comparison : 



Table II. 

 Anode Arc 



Material Mass Gas Pres. Length. Voltage Current Watts 



C 1-3 gr. 18 cm 1 mm 214 0-35 75 



Ag 3-0 21 5 100 1-4 140 



Pb 4-8 30 8 15 1-4 105 



Ag 3-0 31 3 82 1-6 131 



In Table II, the voltage and current are those on the first 

 stage, just before the critical point. The last column gives the 

 total number of watts expended in the arc. The anode was of 

 approximately the same size and shape in each case, and sup- 

 ported in the same way in the apparatus described in §22. For 

 each pair of observations the gas pressure is nearly the same. 

 In the case of silver, the bell- jar contained air ; with lead and 

 carbon, nitrogen. Still, the silver did not oxidize perceptibly, 

 and unless the oxygen changed the electrical conditions through 

 catalytic action, the comparison holds good. 



Comparing silver with carbon and also with lead, the expend- 

 iture of energy is less for the substance of lower heat conduc- 

 tivity. Lead also melts at a lower temperature than silver. 

 The constants of prime importance here are, of course, the 

 boiling points of silver and lead. The comparatively small 

 current at which the critical point occurs in the case of carbon 

 in spite of its high boiling point can only be explained by its 

 low conductivity and on the assumption that the nature of the 

 surface of the anode, as regards roughness and the presence of 

 impurities, plays a very large part. It may be said of the arc 

 in general that far too few trustworthy data have been assem- 

 bled for sweeping conclusions to be drawn as to the effect of 

 the physical properties of the electrodes. 



IIT. The Characteristic Carves of the Iron Arc. 



§30. In fig. 5 is shown a system of curves connecting cur- 

 rent with voltage across the arc for various lengths. The 

 supply e.m.f. was 240 volts, electrodes 4 mm in diameter. For 

 the sake of distinctness, the observations for 1, 4, and 8 mm are 

 represented by dots, those for 2 and 6 mm by crosses. 



The meaning of the lines AjB, CD was referred to in §4, 

 in which the overlapping of the curves on the two stages was 



