sheath. This is a singular and apparently paradoxical view, but it is well founded" (Lodge). And even as to the power of a wire to conduct whatever it does conduct, a special feature has risen into considerable prominence. The most important principle for many years in the study of electricity has been Ohm's law, which states that the resistance of a conductor may be measured by the ratio of the electro-motive force to the current strength. This law when first enunciated was scrutinized closely, demurred against by some experimenters, and shown mathematically to be impossible if carried to extreme applications; it was re-established and experimentally and mathematically proved correct, chiefly by Kirchhoff's work; and is now known to be inaccurate as an expression of the effect transmitted (or resisted) by a conductor under rapid alternations of current, so that to express the energy transmitted under such circumstances another factor has to be taken into account besides what is usually regarded the resistance. This additional quality is called the impedance, and the total resistance of a circuit carrying periodic currents is made up of the ohmic resistance and the impedance. The latter has no value when the current is steady, but has reference only to the time while the current is rising from zero to its maximum strength. The principle of impedance was known a good while ago, but it has only demanded the attention of electricians since the alternating currents have begun to be employed on any considerable scale. Ohm's law is just as true as it ever was, but the limitations of its applicability are now better recognized than formerly.
A rapid succession of electric discharges sets up strains and relaxations in a non-conducting medium, which result in the propagation of waves of electro-magnetic induction through it. With oscillations of great frequency, the waves become short enough to be observed and measured readily, and the recent experiments of Hertz show so many features of similarity in the laws and phenomena of reflection, refraction, and speed of transmission of these waves and of light as to sustain Maxwell's theory of the electromagnetic character of light.
Advances in science are often the outcome of efforts to apply its principles in the arts. A great problem of physics which engineers have to solve is to find economical means of utilizing the energy that Nature is ready to furnish in place of the present wasteful ones. The inefficiency of the best steam engine is a standing reproach to an inventive age. The reproach is to be removed not by the improvement of the steam engine—for its limitations are such that, in the nature of things, it can not be highly efficient—but by the substitution of a better type of machine. Ether vibrations bring us energy in the form of heat, light, or electricity, ac-
