One or more electronic voting machines which shall contain the ballot labels, showing the party emblems and title of officers to be voted for, and which shall, so far as practicable, contain the names of the candidates to be voted for, shall be placed on public exhibition in a suitable place by the board of elections, in charge of competent instructors, for at least fifteen days during the thirty days preceding an election. No electronic voting machine which is to be assigned for use in an election shall be used for such purpose after having been prepared and sealed for the election. During such public exhibition, the counting mechanism of the machine shall be concealed from view and the doors may be temporarily opened only when authorized by the board or Supervisor of Elections. Any voter shall be allowed to examine such machine, and upon request shall be instructed in its use.