D, Diameter. _r_, Radius. _t_, Temperature. θ, Deflection of galvanometer needle. N, _n_, Number of anything. π, Circumference ÷ diameter = 3.141592. ω, 2π_f_ = 6.2831 × frequency, in alternating current. ~, _f_, Frequency, periodicity, cycles per second. φ, Phase angle. G, Galvanometer. S, Shunt. N, n, North pole of a magnet. S, s, South pole of a magnet. A.C. Alternating current. D.C. Direct current. P.D. Pressure difference. P.F. Power factor. C.G.S. Centimeter, Gramme, Second system. B.&S. Brown & Sharpe wire gauge. B.W.G. Birmingham wire gauge. R.p.m. Revolutions per minute. C.P. Candle power. [--o--], Incandescent lamp. [--X--], Arc lamp. [Symbol: Condenser] OR [Symbol: Condenser] Condenser. [Symbol: Battery of cells] Battery of cells. [Symbol: Dynamo] Dynamo, or direct current motor. [Symbol: Alternator] Alternator, or alternating current motor. [Symbol: Converter] Converter. [Symbol: Static transformer] Static transformer. [Symbol: Inductive resistance] Inductive resistance. [Symbol: Non-inductive resistance] Non-inductive resistance. CHAPTER I ELECTRICITY =Nature and Source of Electricity.=--What is electricity? This is a question that is frequently asked, but has not yet been satisfactorily answered. It is a force, subject to control under well known laws. While the nature and source of electricity still remain a mystery, many things about it have become known, thus, it is positively assured that electricity never manifests itself except when there is some mechanical disturbance in ordinary matter. The true nature of electricity has not yet been discovered. Many think it a quality inherent in nearly all the substances, and accompanied by a peculiar movement or arrangement of the molecules. Some assume that the phenomena of electricity are due to a peculiar state of strain or tension in the ether which is present everywhere, even in and between the atoms of the most solid bodies. If the latter theory be the true one, and if the atmosphere of the earth be surrounded by the same ether, it may be possible to establish these assumptions as facts. The most modern supposition regarding this matter, by Maxwell, is that light itself is founded on electricity, and that _light waves_ are merely _electromagnetic waves_. The theory “that electricity is related to, or identical with, the luminiferous ether,” has been accepted by the most prominent scientists. But while electricity is still a mystery, much is known about the laws governing its phenomena. Man has mastered this mighty force and made it his powerful servant; he can produce it and use it.