If you vigorously knead a lemon to free the juices and then stick a
strip of zinc in one end and a copper strip in the other, you can
measure a voltage across the strips. Electrons will flow through the
load without the inconvenience of having to supply heat. You have made
yourself a chemical battery.
The chemical battery was the first direct conversion device. Two hundred
years ago it was the scientists’ only continuous source of electricity.
Since the chemical battery does not need heat for its operation, it is
logical to ask what makes the current flow. Where does the energy come
from?
The battery has no semiconductors, but, like the thermoelectric couple
and the thermionic diode, it uses dissimilar materials for its
electrodes. A conducting fluid or solid is also present to provide for
the passage of current between the electrodes. In the example of the
lemon, the copper and zinc are the dissimilar electrodes, and the lemon
juice is the conducting fluid or _electrolyte_ that supplies positive
and negative ions. The battery derives its energy from its complement of
chemical fuel. The voltage difference arises because of the different
strengths of the chemical bonds. The chemical bond is basically an
electrostatic one; some atoms have stronger electrical affinities than
others.