488 Dr. Norman Campbell on the "Break " 



the potential rises after the separation of the contacts, the 

 shorter will be the time which elapses before the arc is 

 extinguished by the cooling of the terminals ; indeed, if it 

 rises sufficiently slowly the terminals will have cooled 

 before the potential has reached a value which is sufficient 

 to give a "flash," even if the electrodes were hot. In that 

 case the break will be sparkless. On the other hand, it is 

 known that the potential necessary to maintain an arc does 

 not increase rapidly with the distance between the electrodes ; 

 and we should expect, therefore, as we find, that the main- 

 tenance of an arc for an appreciable time, which is probably 

 necessary for the appearance of the "flash," would depend 

 very greatly on the rate of rise of potential and very little 

 on the rate of separation of the terminals. 



A further examination of the curves produces evidence in 

 support of the conclusion that the final cessation of the arc 

 depends on the time which elapses since the break as well 

 as on the distance to which the electrodes have separated. 

 For though the time the arc lasts is longer for the slower 

 separation, the distance of the electrodes when it ceases is 

 greater for the quicker separation (0"30 mm. as against 0*19). 

 Since the potential when the arc ceases is almost the same 

 in the two cases, this difference can only mean that the time 

 as well as the distance is important. The maintenance of 

 the arc retards the cooling of the electrodes, but since the 

 potential difference is not sufficient to keep the electrodes 

 permanently hot, the instant when the arc ceases is determined 

 in part by the losses due to radiation and conduction. 



Again, we can obtain a minimum estimate of the speed 

 of break which would be necessary to obtain a sparkless 

 break simply by increasing the speed of separation. In order 

 that this result should be attained, the separation must be so 

 rapid that the distance between the electrodes becomes so 

 great that, even if the electrodes are still hot, a visible 

 discharge cannot be produced with the potential available. 

 Now curve B shows that the arc can be maintained with a 

 potential of 10 volts with the electrodes 0'30 mm. apart, 

 if they are sufficiently hot ; accordingly mere speed will 

 not prevent sparking unless the electrodes have separated 

 0'30 mm. before curve A has reached 10 volts (or less). 

 The minimum estimate of the speed which will prevent 

 sparking is, therefore, 30,000 cm. /sec. — a value very difficult 

 to obtain in any mechanical break, though much less than 

 the speed of Lord Rayleigh's pistol bullet. However, it 

 must be remembered that the estimate is on all grounds 

 merely a minimum estimate, and there is nothing to render 



