It was discovered that chromic anhydride gives up oxygen easier than nitric acid and consequently if used would give a higher voltage than Grove’s nitric acid battery. It also has the advantage of a lesser tendency to attack zinc directly if it happens to come in contact with it. Grenet developed a cell having a liquid consisting of a mixture of potassium bichromate (K_{2}Cr_{2}O_{7}) and sulphuric acid. A porous cell was therefore not used to keep the two liquids apart. This had the advantage of reducing the internal resistance. The chemical reaction was: K_{2}Cr_{2}O_{7} (potassium bichromate) + 7H_{2}SO_{4} (sulphuric acid) + 3Zn (zinc) = 3ZnSO_{4} (zinc sulphate) + K_{2}SO_{4} (potassium sulphate) + Cr_{2} (SO_{4})_{3} (chromium sulphate) + 7H_{2}O (water). In order to prevent the useless consumption of zinc on open circuit, the zinc was attached to a sliding rod and could be drawn up into the neck of the bottle-shaped jar containing the liquid. [Illustration: GROVE’S INCANDESCENT LAMP, 1840. Grove made an experimental lamp, using platinum for the burner which was protected from draughts of air by a glass tumbler.] DE MOLEYNS’ INCANDESCENT LAMP Frederick De Moleyns, an Englishman, has the honor of having obtained the first patent on an incandescent lamp. This was in 1841 and his lamp was quite novel. It consisted of a spherical glass globe, in the upper part of which was a tube containing powdered charcoal. This tube was open at the bottom inside the globe and through it ran a platinum wire, the end below the tube being coiled. Another platinum wire coiled at its upper end came up through the lower part of the globe but did not quite touch the other platinum coil. The powdered charcoal filled the two coils of platinum wire and bridged the gap between. Current passing through this charcoal bridge heated it to incandescence. The air in the globe having been removed as far as was possible with the hand air pumps then available, the charcoal did not immediately burn up, the small amount consumed being replaced by the supply in the tube. The idea was ingenious but the lamp was impractical as the globe rapidly blackened from the evaporation of the incandescent charcoal. [Illustration: DE MOLEYNS’ INCANDESCENT LAMP, 1841. This consisted of two coils of platinum wire containing powdered charcoal operating in a vacuum. It is only of interest as the first incandescent lamp on which a patent (British) was granted.] EARLY DEVELOPMENTS OF THE ARC LAMP It had been found that most of the light of the arc came from the tip of the positive electrode, and that the charcoal electrodes were rapidly consumed, the positive electrode about twice as fast as the negative. Mechanisms were designed to take care of this, together with devices to start the arc by allowing the electrodes to touch each other and then pulling them apart the proper distance. This distance varied from one-eighth to three-quarters of an inch.