An election shall be held to authorize the issuance of any bonds of a district. The board of directors of a district may call such election at its discretion, and it shall call such election upon presentation to it of a petition requesting the issuance of bonds, specifying the purpose to which the proceeds are to be applied, and signed by electors of the district entitled to cast votes equal in number to at least 15 per cent of the total number of votes of all the electors of the district.