362 ANNUAL REPORT SMITHSONIAN INSTITUTION, 1913. 



When an arc is struck between the electrodes it is at once deflected 

 in a direction perpendicular to the lines of force, and the necessity 

 of having alternating current applied to the electrodes will be ap- 

 preciated from the fact that with direct current the arc would be 

 deflected to one side only. As each electrode is alternatively positive 

 and negative, the arc is projected outward first to one side and then 

 to the other, thus giving a disk of flame about 6 feet in diameter. 

 The speed at which the arc moves outward is extremely rapid, and 

 as the formation of a new arc is practically instantaneous, it appears 

 to the eye as a sheet of flame. 



When the extremities of the arc retire along the electrodes the arc 

 increases in length, its resistance also increasing, until the tension is 

 such that a new arc strikes between the points of the electrodes. The 

 resistance of this short arc being smaller, the tension of the electrodes 

 suddenly sinks to a point that will not sustain the long arc, which is 

 thus extinguished. Another arc starts, and so the process goes on. 



An inductive resistance is a very necessary piece of apparatus to 

 have in series with the arc, because its self-induction automatically 

 effects a displacement of phase according to the currents flowing, 

 thus enabling the arc to burn steadily. 



The writer assisted Mr. Howies with some exp3riments in fixation 

 by nitrogen about 13 years ago, and it was then that the necessity of 

 having an induction coil in circuit was noted. Without it the arc 

 could not be maintained steady, but with it the arc was quite steady. 

 The experiment was made at Messrs. Johnson and Phillips's, Old 

 Charlton, and a transformer that happened to be handy was used for 

 the purpose. 



It should be noted that any furnace working with alternating cur- 

 rent has necessarily a considerable phase difference. In other words, 

 the power factor is low, and therefore, in estimating the sizes of 

 dynamos and cables, due allowance has to be made. This, of course, 

 raises the cost of electric energy. For ordinary power supply a 

 power factor of 0.85 is quite usual, but with fixation of nitrogen fur- 

 naces the power factor is only about 0.6. 



A curious feature of the arc flame is that it is not quite concentric. 

 When looked at through colored glasses the extremities of the arc 

 appear like glowing spots upon the sides of the electrodes; on the 

 positive electrode they are small and fairly close together, whilst on 

 the negative electrode they are larger and farther apart. The reason 

 for these spots appears to be that the arcs solder themselves, so to 

 speak, to the electrodes, and the magnetic lines of force make the 

 extremities of the arcs move along in leaps. For some reason not yet 

 explained, the extremities of the arc cling more closely to the negative 

 than to the positive electrode, and therefore the flame extends farther 



