Science, Maths & Technology

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Introducing engineering

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# 5.3 An inventive step – a 'battery' of cells

Having shown that electrical energy could be harnessed from dissimilar metals in contact, Volta then took his discovery forward a crucial step. He demonstrated that a pile of bimetallic discs such as the ones in Figure 83 would generate electrical energy in proportion to the number of discs in the stack. Increasing the number of discs in the stack increased the amount of energy generated.

Essential to Volta's electric pile was a moisture-bearing layer between the pairs of discs. This arrangement was a development of a horizontal arrangement in which a number of electrochemical cells are connected in series; the 'moisture-bearing layers' were a convenient way to enable a vertical stack of cells to be built. Volta recorded his work in a letter to the Royal Society of London in 1800.

In modern terms we can recognise that the pile is a battery of electrochemical cells placed in series so the electromotive force (e.m.f.) of each cell adds to the e.m.f. of the cell before it – thus making it possible to produce giant batteries. Some 3500 cells in series could drive a continuous stream of sparks through a half-millimetre gap between spheres connected to each end of the pile, for several hours on end.

Figure 82 A voltaic pile: silver and zinc discs separated by layers of moist cloth pads

## Activity 40 (exploratory)

Identify 'an inventive step' in Volta's work on generating sparks from the contact of dissimilar metals.

In fact two inventive steps can be identified. The first is the construction of the pile to form an additive combination of individual electrochemical cells. The second is the use of moist pads to join successive elements.

So where was the necessity that brought about the invention of the battery? Well, beyond the need to satisfy curiosity, there was none. The battery had to mark time while applications were developed. One of the first major uses was in telegraph systems, such as that patented in 1837 by Cooke and Wheatstone (Figure 83). Another early application of 'galvanic electricity', as it was called, was for the remote detonation of explosives by means of an electrically-heated fuse wire. Mining engineers and military engineers alike must have been relieved to discover that explosions could be precisely and safely triggered from a considerable distance.

Figure 83 Wheatstone's telegraph

Shortly after this, a major industrial process turned to electricity to revolutionise the manufacture of domestic artefacts, by electroplating silver onto the surface of otherwise unattractive, but cheap and easily formed metal. However, the full width of the range of uses for electricity was not at first recognised. This extract from an article written in 1851 about the 'The Science of the Exhibition' by Robert Hunt, Esq., Keeper of Mining Records, Museum of Practical Geology is pessimistic about the future applications of electricity:

Although satisfied that, with our present knowledge of electrical forces, we can scarcely hope to adapt the electric light to any useful purpose, within the limits of any ordinary economy, or to apply electro-magnetism as a motive power; it is quite possible that we may, by a careful study of the primary laws of these forms of electrical force, arrive at new conditions which may enable us to apply them. The empirical mode of proceeding at present adopted is of the most hopeless character. The models of electro-magnetic engines exhibited have much in them which is exceedingly ingenious; but, although working well as models, they do not promise to work with regularity or economy on the large scale; and for the present we must rest content to burn coals in our furnace rather than zinc in our batteries.

It is just as well that these discouraging words were not taken to heart.

And so, telegraphers, mining engineers and electroplaters led the demand for more reliability and greater capacity from batteries; battery technology entered a development phase which still continues today.

From the 1870s electric bells caught the imagination of consumers. About 30 years later electric torches were introduced. These needed portable, non-spillable battery units. Since then, any electrical device seems to have been rendered more versatile and more mobile by ever more ingenious electrochemical schemes.