The Daniell cell is a kind of copper-zinc battery which employs a porous barrier between the two metals. It was invented in 1836 by British chemist John Frederic Daniell. Once used widely in the European telegraph industry, it was supplanted in the late 19th century by more modern battery designs. Today, it is primarily used in the classroom to demonstrate how batteries work.
Italian scientist Alessandro Volta invented the battery in 1800. His design used a column of alternating zinc and copper discs, with brine-saturated pieces of cardboard stacked between each piece of metal. This Voltaic pile was used in many electrical experiments, but with less than an hour of battery life, had no real industrial applications. Daniell modified Volta's design to create a battery with a longer life, allowing it to be put to practical use.
The principle behind Daniell's battery and Volta's is the same. A liquid solute called the electrolyte — in Volta's design, the brine between the metal plates — begins to dissolve the zinc and the copper into positively-charged ions. As the ions leave the metal, free electrons remain behind. The zinc dissolves more rapidly than the copper, which means that the zinc soon contains more electrons. If one connects the two pieces of metal with a wire, the electrons will migrate through the wire from the zinc to the copper, creating an electric current.
If the zinc and copper are dissolving in the same electrolyte, as in the Voltaic pile, two processes will shorten the life of the battery. If it is being stored without electricity being drawn, then the copper ions in the electrolyte will be drawn to the negatively-charged zinc terminal. These ions will then couple with the electrons in the zinc, neutralizing its charge. Eventually, no more zinc will be available for dissolution. This process is known as reduction.
The other battery-killing process occurs when electricity is being drawn. The zinc ions will push hydrogen in the electrolyte to the copper, where the hydrogen builds up on the surface and eventually halts the flow of electricity. This process is known as polarization. The Daniell cell was designed in response to the twin problems of zinc reduction and polarization. It solves these problems by isolating the zinc and copper in separate electrolytes.
To make a Daniell cell, an unglazed earthenware container can be filled with sulfuric acid and a piece of zinc immersed in it. The container can then be placed inside a copper can which has been filled with copper sulfate. The earthenware barrier keeps the zinc and copper on opposite sides; this keeps the zinc ions from sending hydrogen to the copper, preventing polarization. It also keeps the copper ions away from the zinc, preventing zinc reduction. Daniell referred to his cell as the constant battery because of its prevention of polarization.
The pores in the barrier do allow positively-charged sulfate ions to travel from the copper side to the zinc side. This balances the flow of electrons from the zinc to the copper when the circuit is completed. The amount of electrical potential produced by a Daniell cell was named the volt. The value of the modern volt is slightly different; a Daniell cell produces about 1.1 of today's volts and has an internal resistance of about 2 ohms.