Section 20. The district attorneys may each employ additional legal assistants. The length of time of such employment shall in no instance exceed six months. The amount of compensation shall be determined by the district attorney providing that for legal assistants who devote their entire time to the duties of legal assistants and do not, either directly or indirectly, engage in the private practice of law said compensation shall in no instance exceed ten thousand dollars and for part-time legal assistants shall in no instance exceed five thousand dollars. Such compensation shall be paid, subject to appropriation, by the commonwealth upon presentation of bills approved by the district attorney. In matters connected with the work for which he is so employed a legal assistant shall have all the powers and authority of an assistant district attorney.