California Evidence Code Section 646
(a) As used in this section, "defendant" includes any party
against whom the res ipsa loquitur presumption operates.
(b) The judicial doctrine of res ipsa loquitur is a presumption
affecting the burden of producing evidence.
(c) If the evidence, or facts otherwise established, would support
a res ipsa loquitur presumption and the defendant has introduced
evidence which would support a finding that he was not negligent or
that any negligence on his part was not a proximate cause of the
occurrence, the court may, and upon request shall, instruct the jury
to the effect that:
(1) If the facts which would give rise to res ipsa loquitur
presumption are found or otherwise established, the jury may draw the
inference from such facts that a proximate cause of the occurrence
was some negligent conduct on the part of the defendant; and
(2) The jury shall not find that a proximate cause of the
occurrence was some negligent conduct on the part of the defendant
unless the jury believes, after weighing all the evidence in the case
and drawing such inferences therefrom as the jury believes are
warranted, that it is more probable than not that the occurrence was
caused by some negligent conduct on the part of the defendant.