Voltaic cells, Electromotive force of cells and Electromagnetic induction

Voltaic cells

             Volta developed the voltaic cell about 1792, and presented his work March 20, 1800.^[30] Volta correctly identified the role of dissimilar electrodes in producing the voltage, but incorrectly dismissed any role for the electrolyte.^[31] Volta ordered the metals in a 'tension series', “that is to say in an order such that any one in the list becomes positive when in contact with any one that succeeds, but negative by contact with any one that precedes it.”^[32] A typical symbolic convention in a schematic of this circuit ( –||– ) would have a long electrode 1 and a short electrode 2, to indicate that electrode 1 dominates. Volta's law about opposing electrode emfs means that, given ten electrodes (for example, zinc and nine other materials), which can be used to produce 45 types of voltaic cells (10 × 9/2), only nine relative measurements (for example, copper and each of the nine others) are needed to get all 45 possible emfs that these ten electrodes can produce.^{[citation needed]}

 Electromotive force of cells

                   The electromotive force produced by primary and secondary cells is usually of the order of a few volts. The figures quoted below are nominal, because emf varies according to the size of the load and the state of exhaustion of the cell.

Emf	Cell chemistry
1.2 V	nickel-cadmium
1.2 V	nickel–metal hydride
1.5 V	zinc–carbon
2.1 V	lead–acid
3.6 V to 3.7 V	lithium-ion

Electromagnetic induction

                 The principle of electromagnetic induction, noted above, states that a time-dependent magnetic field produces a circulating electric field. A time-dependent magnetic field can be produced either by motion of a magnet relative to a circuit, by motion of a circuit relative to another circuit (at least one of these must be carrying a current), or by changing the current in a fixed circuit. The effect on the circuit itself, of changing the current, is known as self-induction; the effect on another circuit is known as mutual induction.

For a given circuit, the electromagnetically induced emf is determined purely by the rate of change of the magnetic flux through the circuit according to Faraday's law of induction.

An emf is induced in a coil or conductor whenever there is change in the flux linkages. Depending on the way in which the changes are brought about, there are two types: When the conductor is moved in a stationary magnetic field to procure a change in the flux linkage, the emf is statically induced. The electromotive force generated by motion is often referred to as motional emf. When the change in flux linkage arises from a change in the magnetic field around the stationary conductor, the emf is dynamically induced. The electromotive force generated by a time-varying magnetic field is often referred to as transformer emf.