It shall be the duty of the board, on the fifteenth day of January of each year, to prepare, in writing, a budget estimating the sum of money which will be necessary for the next fiscal year, to enable the board to discharge the duties hereby imposed upon it, and to meet the expenses of the police department, which it shall forthwith certify to the governing body of such cities, and the budget shall itemize purposes of expenditure by organization units, activities, functions, and character classes in not less detail than “personal services”, “contractual services”, “commodities”, and “capital outlays”, and shall in any event be prepared in form and detail similar to the form and detail in which budgets for the various departments of such city government are prepared. The governing body of the cities is hereby required to appropriate the total amount so certified, payable out of the revenue of the cities after first having deducted the amount necessary to pay the interest on the indebtedness of the cities, the amount necessary for lighting the city, and any sum required by law to be placed to the credit of the sinking fund of the cities, and if the board shall be required to call out extra police force and the expense thereof shall not have been contemplated in their estimate for the fiscal year during which the extra police force is called out, it shall immediately certify the expense of such additional force, and the additional amount so required shall be appropriated for that purpose, except that in no event shall the governing body of the cities be required to appropriate for the use of the police board in any fiscal year an amount in excess of one-fourth of the general revenue fund of such year.