546 THE BELL SYSTEM TECHNICAL JOURNAL, MAY 1954 



equal number of ions obtained by electron-atom collision in the arc 

 column. 



c. Arc Termination 



In general, the arc may be terminated by disturbing one or more of 

 the steady state conditions discussed above. For instance, if the potential 

 across the contacts is decreased to or below the ionization potential of 

 the metal atoms, the necessary ionization process will stop and a de- 

 ficiency of ions in the arc will result. The negative space charge will 

 immediately upset the arc potential distribution interrupting the high 

 electron emission, etc. The arc is also interrupted when the current drops 

 to the minimum arcing current value. This is a well established experi- 

 mental characteristic of the arc which has yet to be explained in terms 

 of the more basic concepts. It is thought, however, that a decreasing 

 arc current decreases the pressure and the atom density in the arc col- 

 umn. It is possible that when a limiting current is reached the ionization 

 rate becomes too small to maintain the condition of equal space charges 

 in the arc column. One should expect, accordingly, that providing the 

 contact surfaces* with a film of low evaporation energy should furnish 

 a more adequate supply of atoms to the arc which may then be main- 

 tained at lower currents. This is in accordance with observations ob- 

 tained for active contacts. 



Arc Initiation Under Dynamic Conditions; Observations on Break 



It appeared of interest to examine the relations between arc initiation 

 voltage and contact separation during the break transient and compare 

 them with measurements made under static coditions. In Fig. 3, the 

 increase in arc initiation voltage with separation is in accordance with 

 the static relation shown as a broken line. During the period 2-3, the 

 breakdowns occurred along longer paths than the minimum contact 

 separation and at the minimum value of the sparking potential. By 

 measurement ^3 = 20 X 10~ sec, S3 = 8 X 10~ cm and ps = 0.61 mm 

 Hg X cm. This is roughly the ps value at the minimum sparking poten- 

 tial in air.^". 



By gradually decreasing the charging times of the transient, by adjust- 

 ing circuit parameters, it was observed that a point was generally 

 reached when a portion of the breakdowns was initiated at voltages well 

 below the coresponding static initiation voltages. Fig. 9 illustrates this 



* The necessarj^ atoms may be obtained from either electrodes or both. Arc 

 transfer observations generally indicate signs of evaporation from both electrodes. 



