E. L. Nichols — Alternating Electric Arc. 3 



" The behavior of the arc, as its length is increased, is very 

 curious. As the point is withdrawn the arc forms and sings 

 with an even tone, the pitch corresponding to the number of 

 alternations. The point becomes of a dull red color while the 

 galvanometer gives a small but quite steady deflection. As 

 the arc is drawn out it sings louder and more harshly, the point 

 becomes redder, while the galvanometer deflection increases and 

 becomes very unsteady. At a certain critical length the follow- 

 ing phenomena suddenly occur : the tone becomes smooth and 

 even, the point brightens almost to a white heat, the intensity 

 depending upon the strength of the current, while the galvan- 

 ometer deflection becomes much greater and very steady. The 

 explanation suggested, and which subsequent experiments seem 

 to confirm, is as follows : At first the arc forms both ways, the 

 rapid succession giving the tone. As the arc lengthens the 

 arc still forms from ball to point, but is only intermittent 

 (occasional) from point to ball, giving the unsteady tone and 

 deflection. Finally the distance becomes too great for the arc 

 to form from the point to the ball, while it still passes freely 

 the other way, and the tone and deflection become steady."* 



It was to the conditions existing in the circuit when the 

 critical length of the arc, above mentioned, had been reached, 

 that Messrs. Archbold and Teeple chiefly devoted themselves. 

 The limits between which it was necessary to maintain the arc, 

 were exceedingly narrow, a very slight extension of the striking 

 distance beyond the critical point, resulting in total extinction 

 of the discharge. Small changes in the speed of the machine 

 were sufficient to throw the apparatus out of adjustment, and 

 the arc, once ruptured, would not reappear spontaneously. It 

 could be re-established, however, by the momentary introduc- 

 tion of a bit of metal between the ball and point, or even by 

 the interposition of a candle flame. The complete stability of 

 the discharge was finally secured by driving the dynamo by 

 means of a motor, the latter being supplied from a storage bat- 

 tery. 



The main portion of the investigation consisted in the deter- 

 mination of the periodic changes of electromotive force and 

 current during a complete cycle, when no arc existed, and of 

 the modifications introduced into the curves of potential and 

 current by the discharge between ball and point. Throughout 

 the entire series of measurements, the striking distance was 

 greater than the critical value already defined, a condition the 

 maintenance of which was secured by watching the indications 

 of the galvanometer. 



The instrument used in the measurement of electromotive 

 force was a Thomson mirror galvanometer of ten thousand 



* Archbold and Teeple ; Thesis, p. 5. 



