Electromotive force, also called emf, is the voltage developed by any source of electrical energy such as a battery or dynamo. The electromotive force is the maximum potential difference between two electrodes of a galvanic or voltaic cell. Energy is converted from one form to another in the generator or battery as the device does work on the electric charge being transferred within itself. One terminal of the device becomes positively charged, the other becomes negatively charged. The work done on a unit of electric charge, or the energy thereby gained per unit electric charge, is the electromotive force. Electromotive force is the characteristic of any energy source capable of driving electric charge around a circuit. The word "force" in this case is not used to mean mechanical force, measured in Newtons, but a potential, or energy per unit of charge, measured in volts. In the electrostatic units of the centimetregramsecond system, the unit of electromotive force is the statvolt, or one erg per electrostatic unit of charge. Devices that can provide emf include electrochemical cells, thermoelectric devices, solar cells, photodiodes, electrical generators, transformer and even Van de Graaff generators. In nature, emf is generated whenever magnetic field fluctuations occur through a surface. The shifting of the Earth's magnetic field during a geomagnetic storm, induces currents in the electrical grid as the lines of the magnetic field are shifted about and cut across the conductors. This book entitled Electromotive Force and Measurement in Several Systems is devoted to different sides of electromotive force theory and its applications in engineering science and industry.