Arcing phenomena at electrical contacts as used in communication engineering

The arc phase of electrical contacts of the type used in low-power communication engineering is discussed from the point of view of the fundamental processes which occur both in the arc column and at the electrode surfaces. A theory of electrode erosion due to spark discharges is applied to the case of contacts, and the results of recent experiments on discharge initiation and arc transfer are discussed. Attention is drawn to the practical consequences of the short time in which an arc can be established. It is shown that the state of the surface of the electrodes themselves is important in arc quenching; restriking of the spark or arc as the contacts separate caa be effected by electron emission from thin oxide films or from dust on the surfaces. The influence of the various disposable parameters on the wear of the electrodes is examined; conclusions are drawn concerning the desirable physical properties of the electrode material and the conditions of operation, in order that the wear shall be least. Recent experiments confirm that dust can be extremely effective in spark initiation, and it is suggested that the maintenance of the electrode surfaces clean and dust-free would reduce the tendency for the striking of the spark or arc with separating contacts, and so reduce the value of the parallel capacitance required for discharge suppression. The use of preferred gas atmospheres and the treatment of the electrode surfaces can assist in arc quenching; and the presence of even very slight self-inductance (about 10¿8 henry) in the local contact circuit (values less, perhaps, than are generally realized) can lead to arcing under certain conditions.