No board of supervisors taking advantage of the provisions of this article shall issue any notes, bonds, or other obligations against the county for the purpose of paying or securing the payment of the cost or expense of purchasing, constructing, or otherwise acquiring, extending, or improving any real or personal property to be used, operated, or controlled under the provisions of this article, and the credit of the county shall not be pledged for the obligations arising out of the exercise of the powers contained in this article. All revenue derived from the operation of any property authorized by this article shall be placed in a separate fund, out of which the expenses are authorized to be paid. The board of supervisors shall have the power to pledge, assign, or otherwise hypothecate the net earnings of said property for a period not exceeding twenty-five years for the purpose of securing the payment of the original cost of the purchase and/or construction and/or extension and/or improvement of such property and also to mortgage or otherwise give a lien on any such property in order to secure the payment of any indebtedness on such property. The board of supervisors shall have power to borrow money for the purpose of paying the expenses authorized by this section and to issue negotiable notes or other evidences of indebtedness each of which shall distinctly state on its face that it is payable solely out of the earnings of the electric power system of such county and that the full faith and credit of the county are not pledged for its payment. The board of supervisors shall have the power to mortgage or otherwise give a lien on the electric power system of the county in order to secure any money borrowed for the purposes authorized in this article. The board of supervisors is expressly authorized to enter into such contracts as may be necessary and proper to carry out the provisions of this section.