This chapter shall not apply to the mayor, councilmen, clerk, treasurer, attorney, librarian, police judge, fire chief, health officer, policemen, or street and water commissioner of the city, while engaged in city business; nor to any employee of the city while so engaged and acting under the direction of any of the above named persons; nor to any member of the fire department while engaged in the necessary business of the fire department; nor to users of the city library during library hours; nor to persons having business with the police or the city clerk in their respective offices; nor to persons attending meetings of the city council or its committees.