Electric Arc between a Ball and Point. 125 



refractory material*. It was finally supplanted by a pointed 

 platinum wire, which, although rendered highly incandescent, 

 withstood the temperature of the arc much better than steel 

 had done. 



The following quotation will serve to indicate the conclu- 

 sions reached by the observers in the course of their prelimi- 

 nary experiments with the platinum point : — 



" The behaviour 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 As the point is withdrawn the arc forms and sings 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 colour, while 

 the galvanometer gives a small but quite steady deflexion. 

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

 point becomes redder, while the galvanometer deflexion in- 

 creases and becomes very unsteady. At a certain critical 

 length the following 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 galvanometer deflexion 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 deflexion. Finally, the dis- 

 tance 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 deflexion become steady " f« 



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 



* The attempt to use carbon terminals led to the following observa- 

 tion : — "A carbon pencil substituted for the point gave the same effect, 

 but upon putting the carbon in place of the ball it still acted as a ' point.' 

 If two carbons were used the more pointed one acted as a 'point.' (It 

 was observed that the end of the needle was fused into a ball by the heat 

 of the arc, and would then act as a ' ball ' to the smaller particles of 

 carbon projecting from the end of the pencil.) Two brass balls brought 

 together caused a drifting of the galvanometer-needle from one side to the 

 other, according, it is to be presumed, as the discharge changed the nature 

 of the two surfaces, so that minute points formed on one or the other." 

 (Archbold and Teeple, Thesis, p. 3.) 



t Archbold and Teeple, Thesis, p. 5. 



Phil. Mag. S. 5. Vol. 31. No. 189. Feb. 1891. L 



