Every plumbing contractor shall be or shall employ the services of at least one licensed master plumber who shall have satisfactorily passed examination of the Board of Examiners of the city. No individual for hire shall work at the installation, repair or alteration of general plumbing within the city unless he or she is employed by a licensed plumbing contractor and under the supervision of a master plumber and is licensed as a journeyman plumber; provided, that one apprentice plumber may be employed to assist a master plumber and that one apprentice plumber may be employed for each journeyman plumber. Before an apprentice plumber shall do any work he or she shall obtain a permit free of charge from the City Clerk, who shall record the name and date of issuance of the apprentice permit. It is hereby declared unlawful for any plumbing contractor or other person to hire anyone not having a plumbing license in force to do plumbing in the city, or to permit anyone in his or her employment not having a plumber’s license issued by the city to do plumbing work of any kind within the city or its zoning jurisdiction. It shall also be unlawful for any person to display or expose the sign “Plumbing” or “Plumber” or a sign containing the words of similar import and meaning unless the person be duly licensed and registered by the city in accordance with this chapter.