Section 13. The mayor, chief of police or city marshal of a city in which such reserve force is established may assign the members thereof to duty in said city whenever and for such length of time as said mayor, chief of police or marshal may deem necessary; and when on duty the members of said reserve force shall have all the powers and duties of members of the regular police force of said city. The compensation of the members of said reserve force shall be fixed by the city council.