Section 120. A customer of a corporation subject to this chapter, or such corporation, may apply to the department for an examination and test of any electric meter, demand indicator, so called, and any other device or appliance installed by such corporation upon a customer's premises and used by such corporation to determine the charge to the customer for its service. The department shall forthwith cause such examination and test as in its judgment is practicable and reasonable to be made by a competent and disinterested person, and shall furnish to the corporation and to the customer a certificate of the result and expense thereof. If, upon such examination and test, it appears that the appliance does not register correctly, the department may order the corporation to correct or remove such meter, demand indicator or other device or appliance and to substitute a correct meter, demand indicator or other device or appliance therefor. All fees for examinations and tests shall in the first instance be paid by the person or corporation making application therefor; but if the examination or test is made at the request of a customer, and the meter is found to be incorrect because too fast, the corporation shall pay such fees to the department, to be repaid by it to the applicant. A meter shall be deemed correct for the purposes of this section if it appears from such examination or test that it does not vary more than two per cent from the standard approved by the department. This section shall not authorize or prohibit differential prices for electricity supplied by any such company.