in a solution of copper sulfate contained in a copper can, which acts as the cell's cathode. The use of a porous barrier allows ions to pass through but keeps the solutions from mixing. Gravity cell In the 1860s, a Frenchman named Callaud invented a variant of the Daniell cell called the gravity cell. This simpler version dispensed with the porous barrier. This reduces the internal resistance of the system and, thus, the battery yields a stronger current. It quickly became the battery of choice for the American and British telegraph networks, and was widely used until the 1950s. The gravity cell consists of a glass jar, in which a copper cathode sits on the bottom and a zinc anode is suspended beneath the rim. Copper sulfate crystals are scattered around the cathode and then the jar is filled with distilled water. As the current is drawn, a layer of zinc sulfate solution forms at the top around the anode. This top layer is kept separate from the bottom copper sulfate layer by its lower density and by the polarity of the cell. The zinc sulfate layer is clear in contrast to the deep blue copper sulfate layer, which allows a technician to measure the battery life with a glance. On the other hand, this setup means the battery can be used only in a stationary appliance, or else the solutions mix or spill. Another disadvantage is that a current has to be continually drawn to keep the two solutions from mixing by diffusion, so it is unsuitable for intermittent use. Poggendorff cell The German scientist Johann Christian Poggendorff overcame the problems with separating the electrolyte and the depolariser using a porous earthenware pot in 1842. In the Poggendorff cell, sometimes called Grenet Cell due to the works of Eugene Grenet around 1859, the electrolyte is dilute sulphuric acid and the depolariser is chromic acid. The two acids are physically mixed together, eliminating the porous pot. The positive electrode (cathode) is two carbon plates, with a zinc plate (negative or anode) positioned between them. Because of the tendency of the acid mixture to react with the zinc, a mechanism is provided to raise the zinc electrode clear of the acids. The cell provides 1.9 volts. It was popular with experimenters for many years due to its relatively high voltage; greater ability to produce a consistent current and lack of any