Whenever a majority of the legally qualified electors voting at an election called and held in any city of the first class having a population of more than fifty thousand, as hereinafter provided, shall vote in favor of the issuance of bonds of such city for the purpose of purchasing, constructing or extending a plant or works for the purpose of supplying such city and its inhabitants with water, gas, electric light or heating, or such part thereof as the governing body of said city may deem necessary, it shall be lawful for the governing body of said city, by ordinance duly passed, to provide for the issuance of the bonds so voted, and to purchase, construct and contract for the construction of such waterworks, gas, electric-light or heating plant.