In case the owner or operator of any public utility is engaged in carrying on any other business in connection with the operation of such public utility, the Commission may require the cost of the operation and gross revenues of such joint business to be kept in such form and manner as may be prescribed by the Commission so that the cost of the operation and gross revenues of the public utility may be ascertained.
Laws 1913, c. 93, p. 151, § 4.