Section 59D. The mayor, chief of the fire department or the officer or board having charge of the said department in a city wherein such reserve force is established may assign the members thereof to duty in the said department whenever and for such length of time as he or it may deem necessary. The compensation of the members of said reserve force shall be fixed by the city council. Except as provided in this and the two preceding sections, no provision of law applicable to the regular or permanent fire force of the fire department of a city shall apply to such reserve force or the members thereof except when assigned to duty as aforesaid.