are used to shift the color of silver bromide black-and-white prints towards brown or blue tones, or to increase their stability. Used on sepia-toned prints, gold toners produce red tones. Kodak published formulas for several types of gold toners, which use gold as the chloride. [ 56 ] Automobiles may use gold for heat shielding. McLaren uses gold foil in the engine compartment of its F1 model. [ 57 ] Gold can be manufactured so thin that it appears transparent. It is used in some aircraft cockpit windows for de-icing or anti-icing by passing electricity through it. The heat produced by the resistance of the gold is enough to deter ice from forming. [ 58 ] Electronics The concentration of free electrons in gold metal is 5.90×10 22 cm −3 . Gold is highly conductive to electricity, and has been used for electrical wiring in some high-energy applications (only silver and copper are more conductive per volume, but gold has the advantage of corrosion resistance). For example, gold electrical wires were used during some of the Manhattan Project 's atomic experiments, but large high current silver wires were used in the calutron isotope separator magnets in the project. Though gold is attacked by free chlorine, its good conductivity and general resistance to oxidation and corrosion in other environments (including resistance to non-chlorinated acids) has led to its widespread industrial use in the electronic era as a thin layer coating electrical connectors , thereby ensuring good connection. For example, gold is