Whenever in the opinion of a presiding judge of a court of common pleas or general sessions of any county of this State about to enter upon the trial of a civil or criminal case the trial is likely to be protracted, the court may cause an entry to that effect to be made in the minutes of the court and, immediately after the jury is impaneled and sworn, the court shall direct the calling of one or two additional jurors in its discretion, to be known as alternate jurors. These jurors must be drawn from the same source, in the same manner, have the same qualifications, and be subject to the same examination and challenge as the jurors already sworn.