Wis. Stat. § 906.14
A trial judge's elicitation of trial testimony is improper if the cumulative effect of the judge's questioning and direction of the course of the trial has a substantial prejudicial effect on the jury. Schultz v. State, 82 Wis. 2d 737, 264 N.W.2d 245 (1978). The practice of judicial interrogation is a dangerous one, but does not require that no court should be allowed to call and question a witness prior to completion of the presentation of evidence. State v. Carprue, 2004 WI 111, 274 Wis. 2d 656, 683 N.W.2d 31, 02-2781.