a film of hydrogen bubbles forming on the copper, which steadily increased the internal resistance of the battery (this effect, called polarization, is counteracted in modern cells by additional measures). The other was a phenomenon called local action, wherein minute short-circuits would form around impurities in the zinc, causing the zinc to degrade. The latter problem was solved in 1835 by the English inventor William Sturgeon, who found that amalgamated zinc, whose surface had been treated with some mercury, didn't suffer from local action. Despite its flaws, Volta's batteries provide a steadier current than Leyden jars, and made possible many new experiments and discoveries, such as the first electrolysis of water by the English surgeon Anthony Carlisle and the English chemist William Nicholson. First practical batteries Daniell cell An English professor of chemistry named John Frederic Daniell found a way to solve the hydrogen bubble problem in the Voltaic Pile by using a second electrolyte to consume the hydrogen produced by the first. In 1836, he invented the Daniell cell, which consists of a copper pot filled with a copper sulfate solution, in which is immersed an unglazed earthenware container filled with sulfuric acid and a zinc electrode. The earthenware barrier is porous, which allows ions to pass through but keeps the solutions from mixing. The Daniell cell was a great improvement over the existing technology used in the early days of battery development and was the first practical source of electricity. It provides a longer and more reliable current than the Voltaic cell. It is also safer and less corrosive. It has an operating voltage of roughly 1.1 volts. It soon became the industry standard for use, especially with the new telegraph networks. The Daniell cell was also used as the first working standard for definition of the volt, which is the unit of electromotive force. Bird's cell A version of the Daniell cell was invented in 1837 by the Guy's hospital physician Golding Bird who used a plaster of Paris barrier to keep the solutions separate. Bird's experiments with this cell were of some importance to the new discipline of electrometallurgy. Porous pot cell The porous pot version of the Daniell cell was invented by John Dancer, a Liverpool instrument maker, in 1838. It consists of a central zinc anode dipped into a porous earthenware pot containing a zinc sulfate solution. The porous pot is, in turn, immersed