The qualifications of such commissioners shall be as follows: To be resident citizens of this State for over two years next preceding the election, and qualified voters under the Constitution and laws, and not less than thirty years of age; nor shall such commissioners, or either of them, be, directly or indirectly, interested in any railroad, street railway, traction line, canal, steam boat, pipe line, car line, sleeping car line, car association, express line, telephone or telegraph line, operated for hire, in this State, or out of it, or any stock, bond, mortgage, security, or earnings of any such railroad, street railway, traction line, canal, steam boat, pipe line, car line, sleeping car line, car association, express line, telephone or telegraph line, compress or elevator companies; and if such Commissioner shall voluntarily become so interested, his office shall become vacant; and if any Corporation Commissioner shall become so interested otherwise than voluntarily, he shall, within a reasonable time, divest himself of such interest; and failing to do this, his office shall become vacant. Nor shall any such commissioner hold any other office under the government of the United States, or of this State, or any other state government, and shall not, while such Commissioner, engage in any occupation or business inconsistent with his duties as such commissioner.