Arcing of Electrical Contacts in Telephone 

 Switching Circuits 



Part II — Characteristics of the Sliort Arc 



By M. M. ATALLA 



(Manuscript received May 27, 1953) 



Results are presented of an experimental study of the characteristics of 

 the short arc in air which is the major cause of contact erosion in telephone 

 switching circuits. Measurements were made of the arc initiation voltage, the 

 voltage drop across the arc and the minimum arcing current. The following 

 are the main conclusions: (1) For ''normal" contacts in air, the arc is 

 initiated at a constant field strength of a few million volts/cm up to separa- 

 tions of about 2-3 mean free paths of an electron in air. At larger separa- 

 tions the arc is initiated at the well knoum spark breakdown potentials of 

 air. In vacuum the linear relation holds for larger separations followed by a 

 transition into a square root relation Vai = K{df^. {2) For "clean^* con- 

 tacts in air, no constant field strength line is obtained for separations as low 

 as 1600 A. Instead, the arc is initiated at the spark breakdown potentials of 

 air, possibly due to adsorbed air molecules or due to breakdown along a 

 longer path at the Paschen^s minimum potential. In vacuum, it is specu- 

 lated that the above square root relation will hold. (3) For ''activated" con- 

 tacts and small separations the arc is initiated at a constant field strength of 

 about 0.6 X 10^ volts/ cm. (4) For "normal" contacts the minimum arcing 

 current increases with an increase in the maxiynum current during the arc 

 due to surface contaminations and the arc cleaning action. (3) For arc cur- 

 rents above 1 .5 ampere and energies of the order of thousands of ergs the 

 cathode determines the arc characteristics. 



INTRODUCTION 



The electrical erosion of contacts presents an important problem in 

 the design of telephone switching apparatus. There are several physical 

 phenomena that occur between contacts and contribute to their erosion. 

 The short arc,* which may occur on both make and break of a contact, 



* The short arc is characterized by its constant voltngo, independent of the 

 current, which is of the order of the ionizing potential of the contact material. 



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