Other concentrations may be represented by additional lines. The voltage potential is represented by a vertical axis, which is labeled Eh, where "h" stands for hydrogen. Physics has an even more strict definition for noble metals. Here, it is required that the electronic structure's d-bands are filled. Other uses of iridium include the following: 1. Electrical contacts for spark plugs (due to its resistance to arc erosion); 2. Computer memory devices; 3. Direct-ignition engine (as a catalyst); 4. Radiotherapy (as a source of radiation); 5. X-ray telescopes. In 2007, worldwide demand for iridium reached 3,701 kilograms (119,000 troy ounces). The two coin versions can be distinguished from one another by the number of serrations they have - the proof version has 220, while the bullion has 40 less than the former's. Other nations, such as Canada, Australia, the United States, and the United Kingdom, started producing their own precious metal coins one after the other beginning in 1979. But while platinum is generally resistant to corrosion, it is corroded by certain elements, such as cyanides (potassium cyanide or sodium cyanide), caustic alkalis, sulfur, and any of the five halogens (astatine, bromine, chlorine, fluorine, and iodine). The following lists some of the properties of platinum: General: * Chemical Symbol: Pt * Atomic Number: 78 * Category (as an element): Transition Metal * Group/ Period/ Block (in the Periodic Table): 10/ 6/ d * Atomic Weight: 195.084 g. Because it was first chanced upon in silver mine in South America, platinum was named as such, after the Spanish word "platina", which translates to "little silver". 2. Palladium - William Hyde Wollaston, an English chemist and physicist, discovered palladium in 1803 in samples of platinum ore obtained from South America. It was discovered on February 5, 1869 at Moliagul, a small township in Victoria, Australia about 37 miles west of the city of Bendigo. The discoverers, John Deason and Richard Oates, found the nugget just a couple inches below the surface on a slope in a place that's sometimes called Black Reef. Records have the following details about the Welcome Stranger: Gross weight: 3,523.5 troy ounces (241.61 pounds) Trimmed weight: 2,520 troy ounces (172.8 pounds) Net weight: 2,315.5 troy ounces (158.78 pounds) Measurement: 2 feet (0.61 meter) x 1.02 feet (0.31 meter) For their find, Deason and Oates were paid about 19,068 by the London Chartered Bank (located in the town of Dunolly in Victoria), where they took the nugget.