Section 6F. The justices of the superior court may designate, for a term of three years from the date of such designation, one of the assistant clerks for the county of Worcester, appointed under section five, to perform, under the direction of the clerk of the courts for the county of Worcester, the duties of clerk pertaining to the criminal proceedings in said court, and such other duties as the clerk shall determine, who shall receive from said county, in addition to the salary paid to him as an assistant clerk under section four, a sum equivalent to ten per cent of the salary of the clerk.