3Stt Cady and Arnold — Electric Arc. 



ceases to hiss at a current somewhere below two amperes ; 

 Stein metz* mentions an unlooked-for change in voltage at 

 about one ampere ; and as we shall show later, the darkening of 

 the arc in the neighborhood of the anodef and the appearance of 

 striations in the arc,;}; which several experimenters have noted, 

 may also be referred to the same common cause. Among the 

 names mentioned, Stark and Cassuto seem to have been the 

 only ones who recognized the nature of the phenomenon. 



Hence it seems better to use the term criticcd point in refer- 

 ring to the effect described in this paper, and to substitute for 

 " quiet state " and " hissing state " the terms first stage and 

 second stage respectively. 



I. The Critical Point of the Iron Arc. 



§2. Apparatus. — Among the metals that have been tried in 

 air at atmospheric pressure, iron shows the critical point by far 

 the most easily. No essential difference in any of the phenom- 

 ena was noticed whether the arc was horizontal or vertical, 

 anode above or below. The effects here described are, however, 

 best obtained with the anode down, probably because the 

 anode, which as will be seen is the seat of the critical point, is 

 then free of disturbing influences from the heated vapors. 



Most of the observations on the iron, arc were obtained with 

 a lamp constructed for the purpose, in which each terminal 

 could be raised and lowered by means of a rack and pinion. 

 Horizontal adjustments permitted the alignment of the ter- 

 minals, which were inclosed in a wooden box with glass sides, 

 to protect the arc against air currents. Current and voltage 

 were measured by means of Weston instruments. The direct 

 current mains yielded a supply at either 120 or 240 volts, which 

 could be raised to 475 volts by connecting a storage battery 

 and small dynamo in series. 



The appearance and length of the arc were observed by pro- 

 jecting an image, magnified about ten times, by means of a 

 small lens, onto a mirror, from which it was brought to a focus 

 on a vertical paper scale graduated in millimeters. The scale 

 was mounted close beside the arc, thus enabling one observer 

 to control the arc and record lengths, while a second observer 

 recorded voltage and current. 



The following- method of determining; leno-th of arc also 

 gave good results in cases where it was desirable not to darken 

 the room. A telescope was focused on the image of the arc 



* Tr. Elec. Cong. St, Louis, vol. ii, p. 710, 1904. 



fMatteucci, C. E., xxix, 263, 1849 ; Arons, Ann. Phys., i, 700, 1900 ; Stark, 

 Phys. Zeitschr., v, 81, 1904; Stark and Cassuto, Phys. Zeitschr., v, 264, 1904. 



JGassiot, Pogg. Ann., cxix, 133, 1863; de la Eue and Miiller, Phil. Tr., 

 clxxi, 65, 1879 ; Child, Phys. Eev., xx, 369, 1905. 



