Distinct types of arcs in relay contacts
We shall distinguish short arcs and plasma arcs. Imagine a type of arc with a length s considerably shorter than the radius a c of the cathodic spot. Then, although the flow lines diverge, the anodic spot will be of about equal size as the cathodic spot. When or as long as an arc corresponds to this description it is called a short or plasmaless arc. It will be seen from the power balance equations, (VIII, 16 and 22), that the anodic spot becomes hotter than the cathode spot. Usually, this fact can be used as a distinction mark. It will not be directly the higher temperature that is observed but a higher evaporation rate for the anode material. Of course, the question arises which ratios s/a c belong to short arcs. It will be seen in § 64 that
is likely to be valid in the short arcs.
$$a_c \geqq 10\:s$$
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© Springer-Verlag Berlin Heidelberg 1958