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What Is the Daniell Cell?

Copper sulfate,which is used in making Daniell cells.
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  • Written By: D.M. Abrecht
  • Edited By: Bronwyn Harris
  • Last Modified Date: 07 April 2014
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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.

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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.

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Discuss this Article

Glasshouse
Post 3

@ Fiorite- Claims of an even older battery have arisen, although; there are no physical examples known to exist. These claims are based around reliefs carved into the walls of the temple of Hathor that some Egyptologists claim depict tomb workers holding rudimentary lamps connected to batteries. These claims are completely theoretical, and do not represent a predecessor to the Volta or Daniell battery. All these reliefs really do are increase speculation that the ancient civilizations may have been more technologically advanced than archaeologists think; leaving plenty to be discovered.

Fiorite
Post 2

A German archaeologist unearthed a peculiar clay pot in the sands surrounding Baghdad in 1938. There is still debate as to what the ancient parthians used the clay pot for, or how exactly it works, but there is speculation it might be the first battery. The apparatus dates back about 2000 years, which would make it older than Volta's battery.

Much like Volta's, and later Daniell's, battery, the Baghdad battery creates an electric current with an electrolyte solution and two types of metal. Where they differ is the types of metal. The Baghdad battery consists of an iron rod inserted into a copper flask filled with electrolyte solution. The copper flask is inserted into the clay pot which is filled with solution

Volta is still attributed with developing the first battery since there are no written records as well as some discrepancy surrounding the exact use, discovery, and dating of the battery.

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