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