Michigan Compiled Laws, Chapter 141. Municipal Financing § 141.39

Sec. 9. In any county within the state of Michigan, which may have elected to act under the provisions of Act No. 42 of the Public Acts of 1913,  1 the provisions of said act shall continue in force in said county until said county shall by referendum elect to operate under the provisions of this act. This act shall not be construed to supersede the provisions of any local or special act relating to the establishment or maintenance of a county sinking fund effective in any county of the state of Michigan until such county by referendum shall elect to come under the provisions of this act. Such referendum may be had by resolution of the board of supervisors, either of its own motion or upon the petition of 10 per centum of the electors of the county.