When it is proposed to refund any outstanding bonded indebtedness of a city upon terms which permit any number of the refunding bonds to mature more than forty years from the time the original indebtedness was incurred, the proposition of refunding the indebtedness shall be submitted to the electors of the city at an election held for that purpose, and the assent of two-thirds of the electors voting at such election is necessary to authorize the issuance of the refunding bonds. The election shall be called and held in the manner of other elections under this article.