For the purpose of placing ballots in the frames of a machine, putting it in order and setting, testing, adjusting and delivering the machine, the board of voter registration and elections or other electoral board may employ one or more competent persons, to be known as the custodians of voting machines, who shall be fully competent, thoroughly instructed and sworn to perform their duties honestly and faithfully. For such purpose such persons shall be appointed and instructed at least thirty days before the election and shall be considered as election officers.