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The Daniell cell, a copper-zinc battery with a porous barrier, was invented in 1836 by John Frederic Daniell. It solved the problems of zinc reduction and polarization in Volta’s design by isolating the metals in separate electrolytes. It is now mostly used in classrooms to demonstrate how batteries work.
The Daniell cell is a kind of copper-zinc battery that employs a porous barrier between the two metals. It was invented in 1836 by the British chemist John Frederic Daniell. Once widely used in the European telegraph industry, it was supplanted in the late 19th century by more modern battery designs. Today it is mostly used in the classroom to demonstrate how batteries work.
Italian scientist Alessandro Volta invented the battery in the 1800s. His design used a column of alternating zinc and copper discs, with brine-saturated cardboard pieces stacked between each piece of metal. This voltaic pile has been used in many electrical experiments, but with less than an hour of battery life, it has had no real industrial applications. Daniell modified Volta’s design to create a battery with a longer life, allowing for practical use.
The principle behind Daniell’s battery and Volta’s is the same. A liquid solute called an electrolyte – in Volta’s design, the brine between the metal plates – begins to dissolve the zinc and copper into positively charged ions. When the ions leave the metal, the free electrons are left behind. Zinc dissolves more quickly than copper, which means zinc soon holds more electrons. If you connect the two pieces of metal with a wire, electrons will migrate through the wire from the zinc to the copper, creating an electric current.
If zinc and copper dissolve in the same electrolyte, as in the voltaic cell, two processes shorten the life of the battery. If it is stored without electricity being drawn, the copper ions in the electrolyte will be attracted to the negatively charged zinc terminal. These ions will then pair with the electrons in the zinc, neutralizing its charge. Eventually, there will be no more zinc available for dissolving. This process is known as reduction.
The other process of battery elimination occurs when electricity is drawn. The zinc ions will push the hydrogen in the electrolyte towards the copper, where the hydrogen builds up on the surface and eventually cuts off 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 dipped in a piece of zinc. The container can then be placed inside a copper can that has been filled with copper sulfate. The terracotta barrier holds the zinc and copper on opposite sides; this prevents the zinc ions from sending hydrogen to the copper, preventing polarization. It also keeps copper ions away from the zinc, preventing zinc reduction. Daniell called his cell the constant battery because of its bias avoidance.
The pores of the barrier allow positively charged sulfate ions to travel from the copper side to the zinc side. This balances the electron flow from zinc to copper when the circuit is completed. The amount of electrical potential produced by a Daniell cell has been called a volt. The value of the modern volt is slightly different; a Daniell cell produces about 1.1 of the current volts and has an internal resistance of about 2 ohms.
