During the course of his experiments, Davy noted that when the two terminal wires from a large Voltaic battery were touched together and then drawn apart, not only did a spark pass, but a continuous discharge of great brilliancy, that did not cease until the wires were separated by a considerable distance. The extent of this distance was found later to be dependent on the number of cells in the battery. He noted also that the discharge did not follow a straight line, but was bent into an arc; and for this reason he gave it the name, "Voltaic arc." This light is still known by the name "arc light." Its importance does not seem to have been realized until after the dynamo-machine had been invented, and means thereby supplied for providing a greater amount of electric current, and at less expense than Voltaic cells were capable of delivering. Davy's last great invention was his miner's safety lamp, made in 1816. There had been frequent explosions in the collieries, attended with great loss of life, and Davy was requested to try to ascertain how they could be prevented. After visiting the mines, he had samples of the gas that was found in them sent to him for investigation. He went about the work with scientific thoroughness and system, and ascertained that the gas would not explode if it were mixed with less than six times or more than fourteen times its volume of air; that air rendered impure by the combustion of a candle would not explode the gas; that, if a candle were burnt in a closed vessel, with small openings near the flame, no explosion would take place, even if the vessel were introduced into an explosive mixture; and that the gas from the mines would not explode inside a tube less than 1/8 inch in diameter. These data being secured, Davy conceived the idea of making a lamp in which a small oil light should be fixed and surrounded with a cylinder of wire gauze. He then embodied his conception in a concrete form, and the "Miners' Safety Lamp" resulted. This was an invention of the first order; original, concrete and highly useful. After meeting the customary chorus of prejudice and opposition, it justified its existence by a quickly established record of effectiveness, and took its place among the useful adjuncts of the machine of civilization. Meanwhile, several other adjuncts had appeared. Among these was the steel pen, a process of making malleable iron castings, the planing machine, a fireproof safe, the knitting machine and the band wood-saw.