# The Magnetic Field of the Electric Gun

(Reproduced from "Zero to Eighty" pp.327-329 by EF Northrup)

The simplest and most direct method for obtaining a clear mental picture of the magnetic field produced by an electric gun is to make “field of force pattern,” as shown in Fig. 12. The pattern in this figure was made with iron filings, the various configurations being entirely to the magnetic field produced by the proper value of urrent in each coil. In making the pattern, direct currents were used in the coils, their direction and value being chosen to represent a condition which would exist at a certain selected instant, if polyphase rurrents were actually used. Of course, only direct currents or single-phase currents can be used to make a field of force pattern of this kind; true polyphase currents would not make a pattern because the resulting magnetic fields do not stand still in space.

Taking the pattern to represent a cross-section of the external mag-netic fields of the coils on a plane passing through their axis, it is easy to visualize the entire magnetic field as a series of toroids or “dough-nuts” of magnetic flux. With polyphase currents these toroids move continuously along the gun from left to right, or in the reverse direc-tion, according to the connections used. The inner portion of each magnetic toroid is located inside the coils, while the outer portion is external to the coils as shown.

As the invisible toroids produced by polyphase currents move along the gun, lines of magnetic flux intercept the surface of the projectile and set up other currents; these, reacting on the flux, produce forces which drag the projectile along with the moving magnetic field. However, of the flux which intercepts the surface of the cylindrical projectile, only that portion which has a component perpendicular to the surface is effective for producing motion.

Fig. 9 (page 312), a photograph of the connection side of the model with which the field of force pattern was made, shows the general method of joining the various coils. The method used here con-nects the eighteen individual coils so as to produce three complete waves — a wave being defined as the distance between coils three hundred and sixty electrical degrees apart. This makes each coil differ progressively by sixty electrical degrees from adjacent coils, which accounts for the fact that there are two magnetic toroids per wave-length, representing the positive and negative value of each cycle.

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