Sometime during the 1860s, a Frenchman by the name of Callaud invented a variant of the Daniell cell which dispensed with the porous barrier. Instead, a layer of zinc sulfate sits on top of a layer of copper sulfate, the two liquids are kept separate by their differing densities, often with a layer of oil added on top to prevent evaporation. This reduces the internal resistance of the system and thus the battery yields a stronger current.
This variant, called a gravity cell, consists of a glass jar in which a copper cathode sat on the bottom and a zinc anode is suspended beneath the rim in the zinc sulfate layer. Copper sulfate crystals are scattered around the cathode and the jar then 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. A disadvantage of the gravity cell 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. In addition, it was vulnerable to loss of integrity if too much electric current is drawn, which also causes the layers to mix.
Sometimes called the crowfoot cell due to the distinctive shape of the electrodes, this arrangement is less costly for large multicell batteries and it quickly became the battery of choice for the American and British telegraph networks. Even after most telegraph lines started being powered by motor-generators, the gravity battery continued to be used in way stations to power the local circuit at least into the 1950s. In the telegraph industry, this battery was often assembled on site by the telegraph workers themselves, and when it ran down it could be renewed by replacing the consumed components. The zinc sulfate layer is clear in contrast to the deep blue copper sulfate layer, which allows a technician to determine the battery life with a glance. On the other hand, this setup means the battery could only be used in a stationary appliance, otherwise the solutions would mix or spill.