An elementary book on electricity and magnetism and their applications . uc-tive circuit. Whyis it that when a wire doubles back on itself, as shown in figure223, there is no inductance ? In this, as in all such electrical problems, it is necessaryonly to return to the first principles of the magnetic actionof an electric current. It is easy to see that the magneticflux circulating around one wire will be neutralized by the fluxabout another carrying a current in the opposite we shall have only theohmic resistance of the wireto consider. The resistancecoils used with a Wheat-sto
An elementary book on electricity and magnetism and their applications . uc-tive circuit. Whyis it that when a wire doubles back on itself, as shown in figure223, there is no inductance ? In this, as in all such electrical problems, it is necessaryonly to return to the first principles of the magnetic actionof an electric current. It is easy to see that the magneticflux circulating around one wire will be neutralized by the fluxabout another carrying a current in the opposite we shall have only theohmic resistance of the wireto consider. The resistancecoils used with a Wheat-stone bridge (section 132)are wound filament of an incandes-cent lamp is practically non-inductive. A straight wireis also practically noninduc-tive, although if the wire isof large size, there is a slightinductive effect. 222. Choke coils. A chokecoil is merely a coil havingconsiderable reactance. Figure 224 shows one form of chokecoil such as is used on transmission lines in connection withlightning arresters, which will be described in Chapter Fig. 224. — Hour glass form of chokecoil used in connection with lightningarresters. INDUCTANCE AND CAPACITY 325 The apparatus to be protected is connected at A and the lineat L. If a surge (or big electrical wave due to lightning) travelsalong the transmission line, a very high voltage is induced inthe reactance coil. The coil thereby tends to force the high-frequency lightning current to the ground through the light-ning protector. 223. How to calculate inductance. In the first place it isnecessary to keep in mind that inductance is not a materialthing but merely the name of a certain algebraic expression relat-ing to the shape, size, and dimensions of an electric expression given below, though exactly true only for along solenoid, may be used as an approximation in calculatingthe inductance of ordinary coils. The proof of this algebraicexpression will be found in the more advanced books on alter-
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