Abstract :
It is a common experience that in the brush-collector contact under normal circumstances the contact voltage U = Umin is of the order of 1 volt. In this paper the proof is given that this property is correlated with the strongly resistive collector films through which conductive spots (a-spots) have to be initiated by a kind of electrical breakdown, called fritting. The correlation between Umin and the fritting voltage, Uf, is established as follows. Clean contacts, carbon versus carbon, are characterized by a contact resistance, R, which is given by the load, P, but is independent of the current, I, provided they are not essentially heated by the current. However, in the event of a highly resistive film in the contact a certain voltage is required to electrically break down the film whereby a-spots are generated. It is characteristic of this phenomenon that the a-spots are smaller than the load-bearing area, assuming such sizes that R becomes dependent on I. Clear evidence is given by (RU) curves at various I and P for two kinds of resistive films, that at small I, U approaches Umin. Furthermore, it is proved that Umin is predetermined by Uf of the very film. At larger I, the voltage, ¿Uw, adds to Uf in order to widen the a-spots, adapting them to I. It is shown that ¿Uw is relatively small with the result that U(I) = Uf+¿Uw remains close to Uf, for a wide range of I and P.
