Electrical engineering has developed largely and widely within a very short time and its many applications has created so great a demand for various kinds of electrical apparatus, that their manufacture forms one of the leading industries. Electricity is very valuable as a medium for the transmission of energy, especially to long distances; it is also used to great advantage in lighting, being free from the disagreeable properties of gas or oil. Again, electricity finds various applications, in extracting gold from the ore, pumping and ventilation of mines, traction, telephone, telegraph, electroplating, therapeutics, etc. These few, of its many applications will perhaps serve to indicate the far reaching interest and importance of electricity, and possibly help to kindle in the student something of the eagerness in his work and enthusiasm without which he will fail to do justice either to his calling or to himself. SIGNS AND SYMBOLS The following signs, symbols and abbreviations are almost universally employed in descriptive and technical works on electrical subjects. Although, in the arrangement of the Guides, the direct current and alternating current matter has been kept separate, it is perhaps advisable in the case of signs and symbols, to combine those relating to the alternating current with the direct current and other symbols, making a single table, rather than have them scattered throughout the work. =1. Fundamental.= _l_, Length. cm. = centimeter; in., or ″ = inch, ft. or ′ = foot. M, Mass. gr. = mass of 1 gramme kg. = 1 kilogramme. T, _t_, Time, _s_ = second. =2. Derived Geometric.= S, _s_, Surface. E, Volume. α, β, Angle. =3. Derived Mechanical.= _v_, Velocity. ω, Angular velocity. _r_, Momentum. _a_, Acceleration. _g_, Acceleration due to gravity = 32.2 feet per second. F, _f_, Force. W, Work. P, Power. δ, Dyne. ε, Ergs, ft. lb., Foot pound. H.P., h.p. Horse power. I.H.P., Indicated horse power. B.H.P., Brake horse power. E.H.P., Electrical horse power. J, Joule’s equivalent. _p_, Pressure. K, Moment of inertia. =4. Derived Electrostatic.= _e_, Pressure difference. _i_, Current. _r_, Resistance. _q_, Quantity. _c_, Capacity. _sc_, Specific inductive capacity. =5. Derived Magnetic.=