The study of the spectrum has proven a most fruitful field of endeavor: to the physicist, in tent on knowing the structure of the molecule and the atom, it has suggested some of his best ideas; while to the chemist is given the ability to identify the minutest quantities of elements, by examination of emission, transmission (ab sorption), and reflection spectra; and to the astronomer starlight has revealed, in its spec trum, the. composition, and sometimes the motion, of its celestial source. Photometry, the measurement of the brightness of luminous sources and of illuminated surfaces (illuminom etry), is a branch of the applied science, developed by the physicist and psychologist in close co-operation, which is of great import ance to the physical investigator, as well as to the illuminating engineer.
Electricity and magnetism are nowadays treated as a single subject, although in its earlier stages of development, different types of electric and magnetic phenomena were studied as magnetism, static electricity (electrostatics), and current electricity (electrokinetics), and it was only after considerable, later work that the close relationship between them was recognized, as manifestations of the same form of energy. The division is still made for convenience in many treatises, particularly by those who favor the historical method. Magnetism covers the properties of permanent magnets, and the phe nomena of magnetic fields produced by such magnets. Many of the problems of terrestrial magnetism may be considered on the assump tion that the earth is a huge magnet, without regard to its source of magnetization, and most of the methods of magnetic survey have been developed under this heading. The magnetic properties of paramagnetic and a dimagnetic substances (permeability, hysteresis, etc.) may be considered here, but are more generally included as a part of electrokinetics, under the sub-topic of electromagnetism. The °mole cular* (Ewing's) theory of magnetism is here used to explain the facts, although its complete development also properly belongs to the later section.
The quantity factor of electrical energy is electric charge; the intensity factor is electric potential difference. The presence of electric potential difference across a non-conductor (dielectric) produces what is often described as a piling up on the opposite boundaries of the dielectric of equal and opposite quantities of electric charge (Maxwell's explanation of this, as a momentary displacement of electric charge across the dialectric is believed at present to be a more rational one) ; and this results in a state of strain, described as a field of electrostatic force, throughout the dielectric. The presence of electric potential difference across a con ductor produces a continuous displacement or •flow* of electric charge across the conductor which constitutes an electric current, and which is accompanied by a state of magnetic strain or a field of magnetic force in the conductor and in the surrounding space. Study of the phe
nomena of the electrostatic field constitutes electrostatics, while that of the phenomena ac companying electric flow or current constitutes electrokinetics. The latter topic has proven up to this time far the more important, although the modern physics, both in theory and in prac tice, has been forced more and more to consider the electrostatic phenomena which accompany current flow; as for example, in electro-mag netic radiation theory and its applications in wireless communication, in the theory and the applications of gaseous conduction, and in the design and operation of long distance, high potential transmission lines.
Electrical energy becomes manifest to our senses only indirectly, as it is transformed into other forms of energy, such as heat energy, chemical energy and mechanical energy; the last most often through the intermediary of electro-magnetic force action. And the methods of measurement and of utilization of electrical energy are indirect, measuring and utilizing the transformed energy. On the other hand, elec trical energy is produced by the transformation of the other forms of energy into electrical energy, and the commercial utilization of elec tricity became practicable only after the inven tion of dynamo-electric machinery, which made possible the efficient transformation of mechanical energy into electrical energy; the other methods of production being too limited and costly. Too numerous to mention are the inventions and practical applications of elec tricity to the needs of humanity: in communi cation, from the earliest telephone and the first Atlantic cable to the wireless telegraph and telephone of to-day and to-morrow; in illumi nation, where electricity has revolutionized the world's lighting methods; in power transmis sion, which has made possible the utilization of the least accessible of the world's power re sources; in power application, from the largest motors to the smallest motors for fans and toys; in chemical production — the whole de velopment ofelectro-chemistry; in medicine, as in the applications of X-rays; and in liter ally millions of other ways, in every conceiv able field, electricity is serving humanity. The development of the production, transmission and utilization of electric power constitutes the business of the electrical engineer. A complete outline of this quite modern, but already very extensive subject, with its relations to electro chemistry and electrical engineering, will be found elsewhere in these volumes. The most recent developments of electrical science in the way of advancement of knowledge of the nature of electricity itself are so epochal, so extended and so far-reaching in their bearing on all other physical theory, that their consideration is properly left to the paragraphs that follow.