The electromotive force is, quite literally, the driving element behind many everyday items and systems. These include generators, transformers and solar cells - even the Van de Graff generators that make impressive sparks in horror movies are examples of the electromotive force in action.
The electromotive force is best seen in the battery, and was detailed by Michael Faraday in 1830, following up on work by Alessandro Volta, who invented the device. Strictly speaking, electromotive force is not a force, but rather a measurement of potential expressed in volts. The force acts as a "charge pump" that moves electrons from a source of high potential to one of lower potential, creating voltage. The chemical reactions within the battery as the electrolyte solution interacts with the electrodes, is the basic source for electromotive force as power is created.
The mystery of how electromotive force was generated occupied the minds of scientists through much of the 19th Century. Walter Nernst discovered that the interface between the electrodes and electrolyte was the scene of the creation of the electromotive force. A typical battery consists of dissimilar electrodes surrounded by a suitable electrolyte solution - examples include lead-acid, nickel-cadmium and zinc-carbon. As each of the electrodes react with the electrolyte, ions are charged and pushed toward the dissimilar electrode by the electromotive force. The rate of this flow, measured in volts, is the force itself.
The force is created in generators through the rotation of a time-dependent magnetic field - the rotor of the generator interacting with the stator, which creates electrical energy through the process known as induction. As in the battery, the electromotive force is then the measurement in volts of the power potential. The electromotive force is generated either statically or dynamically, depending on the type of generator used, and is also referred to as transformer electromotive force.