§ 45. Examinations; conducting business without certificate prohibited. A person desiring or intending to conduct the trade, business or calling of a plumber or of plumbing in a city of this state as employing or master plumber, shall be required to submit to an examination before such examining board of plumbers as to his experience and qualifications for such trade, business or calling, and it shall not be lawful in any city of this state for a person to conduct such trade, business or calling, unless he shall have first obtained a certificate of competency from such board of the city in which he conducts or proposes to conduct such business.