Abstract
Direct observation of initiating electrons below the threshold for visible breakdown discharge is reported. With the cloud-chamber techniques used, minimum detectable ionization is estimated at 5000 ion pairs/${\mathrm{cm}}^{3}$. Production of initial electrons is ascribed to photoionization, in a manner which is consistent with Phelp's model. Above the visible breakdown threshold, thermal-gradient effects overcome all other sources of droplet formation in the cloud chamber.
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