5i8 THE POPULAR SCIENCE MONTHLY. 



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 prin- 

 ciple for many years in the study of electricity has been Ohm's 

 law, which states that the resistance of a conductor may be meas- 

 ured 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 mathe- 

 matically 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 in- 

 accurate as an expression of the effect transmitted (or resisted) 

 by a conductor under rapid alternations of current, so that to ex- 

 press the energy transmitted under such circumstances another 

 factor has to be taken into account besides what is usually re- 

 garded the resistance. This additional quality is called the im- 

 pedance, 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 refer- 

 ence 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 elec- 

 tricians 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 recog- 

 nized than formerly. 



A rapid succession of electric discharges sets up strains and 

 relaxations in a non-conducting medium, which result in the propa- 

 gation 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 phe- 

 nomena of reflection, refraction, and speed of transmission of these 

 waves and of light as to sustain Maxwell's theory of the electro- 

 magnetic 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 en- 

 gineers have to solve is to find economical means of utilizing the 

 energy that Nature is ready to furnish in place of the present waste- 

 ful 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 vibra- 

 tions bring us energy in the form of heat, light, or electricity, ac- 



