When an electromagnetic wave impinges on a sheet of metal, the electric field component induces a current flow in the sheet. The alternative is the use of an absorbing material that converts the incident electromagnetic field into thermal energy inside the material. Due to the relatively high surface resistance and thinness of most conductive coatings, neither their shielding effectiveness nor their current-carrying capability is as high as that of pure metal. The greatest contributor to shielding degradation is typically the presence of joints and apertures and not the enclosure, unless the material has low conductivity. One of the weaknesses of the transmission-line theory when applied to shielding is that the shielding effectiveness is dependent, on the distance between the source of the field and the shield, due to the variation in wave impedance with distance. Manufacturers' shielding effectiveness curves for these materials are invariably for electric field attenuation, which is the easiest type of field to shield against.