§ 3. "That, as from the first day of January, nineteen hundred and thirty-one, a motor vehicle shall not, for the purpose of determining the rate of the licence duty chargeable under the Second Schedule to the Finance Act, 1920, be deemed to be electrically propelled unless the electrical motive power is derived either from a source external to the vehicle, or from an electrical storage battery which is not connected to any source of power during the time when the vehicle is in motion."