Petitions provided for in sections 78.010 to 78.420 shall be signed by none but voters of the city. Each petition shall contain, in addition to the names of the petitioners, the street and house number in which the petitioner resides, his age and length of residence in the city. It shall be accompanied by an affidavit of one or more voters of the city stating that the signers thereof were at the time of signing voters of said city and the number of signers at the time the affidavit was made.