In districts in which voting machines are to be used, the county board of elections, or the custodians appointed by them, shall instruct in the use of the machines, and in their duties in connection therewith, all judges and inspectors of election and machine inspectors who are to serve at the primary or election, and who have not been previously instructed and found qualified, and they shall give to each judge, inspector and machine inspector, who has received such instruction and is found qualified to conduct such primary or election with the voting machine, a certificate to that effect. For the purpose of giving such instructions, the county boards shall call such meeting or meetings of election officers as shall be necessary. Each judge, inspector and machine inspector shall, upon notice, attend such meeting or meetings called for his instruction and receive such instruction as shall be necessary for the proper conduct of the primary or election with voting machines, and, as compensation for the time spent in receiving such instruction, each judge, inspector and machine inspector who shall qualify for and serve at such primary or election, shall receive the sum of five ($5.00) dollars, to be paid to him at the same time and in the same manner as compensation is paid to him for his services on election day. No judge, inspector or machine inspector shall serve at any primary or election at which a voting machine is used, unless he shall have received such instructions, shall have been found qualified to perform his duties in connection with the machine, and shall have received a certificate to that effect from the county board or one of the custodians appointed by them: Provided, however, That this shall not prevent the appointment of a judge or inspector of election or machine inspector to fill a vacancy arising on the day of election or on the preceding day.