If it shall be necessary for the attorney for the Commonwealth of any county or city to absent himself for a prolonged period of time from the performance of the duties of his office, then, upon notification by such attorney for the Commonwealth, or by the court on its own motion, and the facts being entered of record, the judge of the circuit court shall appoint an attorney-at-law as acting attorney for the Commonwealth to serve for such length of time as may be necessary. Such acting attorney for the Commonwealth shall act in place of and otherwise perform the duties and exercise the powers of such regular attorney for the Commonwealth, and while so acting shall receive the salary and allowance for expenses fixed by the State Compensation Board for such regular attorney for the Commonwealth, who during such length of time shall not receive any such salary or allowance.