How does an electric eel produce a current

Fundamentally, the eel is simply a living battery. The tips of its head and tail represent the poles of the eel s battery . As much as 80 per cent of its body is an electric organ, made up of many thousands of small platelets, which are alternately super-abundant in potassium or sodium ions, in a similar manner to the potentials formed across axon membranes in nerve cells (see p. 339). In effect, the voltage comprises thousands of concentration cells, each cell contributing a potential of about 160 mV. It is probable that the overall eel potential is augmented with junction potentials between the mini-cells. 

A battery is defined as a device for converting chemical energy into electrical energy. A battery is therefore an electrochemical cell that spontaneously produces a current when the two electrodes are connected externally by a conductor. The conductor will be the sea in the example of the eel above, or will more typically be a conductive 

There are several types of battery we can envisage. A majority of the batteries we meet are classed as primary batteries, i.e. a chemical reaction occurs in both compartments to produce current, but when all the chemicals have been consumed, the battery becomes useless, so we throw it away. In other words, the electrochemical reactions inside the battery are not reversible. The most common primary batteries are the Leclanche cell, as described below, and the silver-oxide battery, found inside most watches and slim-line calculators. 

By contrast, secondary batteries may be reused after regenerating their original redox chemicals. This is achieved by passing a current through the battery in the opposite direction to that during normal battery usage. The most common examples of secondary batteries are the lead-acid cell (there is one inside most cars) and nickel-cadmium batteries (commonly called NiCad batteries). 

In the shops, secondary batteries are usually called rechargeable batteries. 

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