The terms of the first directors of the district to be elected shall be determined in relation to the amount of the taxable wealth in their respective director districts. The candidates of the wealthiest one-third of the total number of director districts shall serve for a term of six years; the candidates of the next wealthiest one-third of the total number of director districts shall serve for a term of four years; the candidates of the next wealthiest one-third or lesser number of the total number of director districts shall serve for a term of two years.