What are the limits of a trial judge's power to examine witnesses to elicit or clarify testimony?

Incident to his duty to conduct proceedings with a view toward the effective ascertainment of truth (Pen.Code, 1044), a trial judge possesses inherent power to examine witnesses to elicit or clarify testimony (e.g., People v. Rigney (1961) 55 Cal.2d 236, 241, 10 Cal.Rptr. 625, 359 P.2d 23). Additionally, we have previously observed that '(t)he mere fact the judge examined . . . at some length does not establish misconduct.' (People v. Corrigan (1957) 48 Cal.2d 551 at p. 559, 310 P.2d 953 at p. 958.) A trial judge may Not, however, in the course of examining witnesses become an advocate for either party or cast aspersions or ridicule upon a witness. (People v. Rigney, supra, 55 Cal.2d at p. 241, 10 Cal.Rptr. 625, 359 P.2d 23, and cases cited therein.) Moreover, he should properly undertake the examination of witnesses 'only when it appears that relevant and material testimony will not be elicited by counsel.' (Id. at p. 243, 10 Cal.Rptr. at p. 629, 359 P.2d at p. 27.) Clearly these latter limitations have been persistently ignored by petitioner. Perhaps the most flagrant instance was his embarcation in one disturbing- the-peace case upon an argumentative dialogue devoted to a wholly irrelevant inquiry about the testifying defendant's illegitimate children and her attitude toward him as a judge.