The power of public service companies to issue stocks and stock certificates or other evidences of interest or ownership, and bonds, notes and other evidences of indebtedness and to create liens on their property situated within this Commonwealth is a special privilege, the right of supervision, regulation, restriction, and control of which is and shall continue to be vested in the Commonwealth; and such power shall be exercised as provided by law and under such rules and regulations as the Commission may prescribe; provided that this section shall not apply to obligations incurred for purchase of machinery or equipment where such obligations are secured by conditional sales contracts.