The invention relates to conductive materials particularly to serve as a heater. Heating means formed by or with electrically conductive materials are known and particularly effective for its flexibility is that sold under the British, Community and US Registered Trade Mark INDITHERM. Conductive rails are used to provide electrical connection to a source of power that need to have a commensurate flexibility, and the object of the invention is to ensure the maintenance of power to the full length of the conductive rail. This objective, is met by a construction comprising spaced first rails for the supply and return of electrical power, the said rails having a flexibility compatible with the semi-conductive material, and there being a supplementary rail attached to each first rail along the length thereof, the supplementary rails being flexible and having strength characteristics greater than those of the first rails.