As soon as practicable after the election the commissioners shall meet and organize as the board of electric light, water supply, fire and sewerage commissioners, as the case may be, for such district. They shall draw by lot the commissioners who shall hold the two, four and six year terms respectively. At the organizational meeting they shall elect one of their number as chairman. In the event of any vacancy on the commission the remaining members of the commission shall elect a commissioner to fill the unexpired term.