What is a trial judge's duty to ensure that the jury is not prejudiced at trial?

When the possibility of prejudice arises during trial, the trial judge "has the affirmative duty ... to take positive action to ascertain the existence of improper influence on the jurors' deliberative qualifications and to take whatever steps are necessary to diminish or eradicate such improprieties." Silverthorne v. United States, 400 F.2d 627, 643 (9th Cir.1968). "[T]he trial judge's delicate estimation of the needs of the case of which he has firsthand experience" will be upheld on appeal unless the judge "has clearly abused his discretion." United States v. Polizzi, 500 F.2d 856, 881 (9th Cir.1974), cert. denied, 419 U.S. 1120, 95 S.Ct. 802, 42 L.Ed.2d 820 (1975).