At the beginning of the nineteenth century, the study of electricity and magnetism were two separate disciplines with separate sets of units. Both sets of units recognized only mass, length, and time as fundamental physical dimensions, because the recent eighteenth-century triumphs of Newtonian physics had predisposed scientists to assume that these three dimensions were the only ones needed to describe nature. The conceptual neatness of separate units disappeared, however, during the first half of the nineteenth century, due to the discovery of ever more profound connections between electrical and magnetic phenomena, a process culminating with Maxwell's identification of light as electromagnetic radiation. Units for the new combined discipline of electromagnetism could be created either by extending the electrical units to cover magnetism, creating a new system called electrostatic units (esu), or by extending the magnetic units to cover electricity, creating a new system called electromagnetic units (emu). We shall see that both procedures gained widespread acceptance, requiring scientists to become familiar with both systems. To make the situation even more confusing, the two systems gave units that depended in different ways on mass, length, and time to the same electromagnetic physical quantities. Consequently, esu and emu units, unlike the units of Newtonian physics, are different in kind as well as size; this inevitably became a source of discontent to those who felt that the same kind of physical quantity should always be measured by the same kind of physical unit.
During the second half of the nineteenth century, the situation began to sort itself out with the growth and adoption of âpracticalâ units based on a rescaled system of emu units. Although these units did not long survive the nineteenth century, historically speaking they paved the way for the most widespread electromagnetic system in use today, the rationalized mks system. In this chapter we not only present the system of practical units but also discuss the growing realization that electromagnetic units could be based on four, rather than three, fundamental dimensionsâ mass, length, and time, and something electromagnetic (for example, charge). The easiest way to show how this all fits together is to sketch in a very old-fashioned, nineteenth-century version of electromagnetic theory, showing how nineteenth-century formalism influenced the development of the first electromagnetic units.
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