3-8
in a helical or corkscrew path shown in figure 3-6. With the proper velocity of the electron and strength of
the magnetic field, the electron will be caused to move at an angle which allows it to converge with other
electrons at some point on the crt screen. Focusing is accomplished by adjusting the current flow through
the focusing coils.
Figure 3-6.—Helical motion of electron through a uniform magnetic field.
The focused electron beam is deflected by a magnetic field that is generated by current flow through
a set of deflection coils, as shown in figure 3-5. These coils are mounted around the outside surface of the
neck of the crt. Normally, four deflection coils (N, S, E, and W) are used, as shown in figure 3-4. Two
coils in series are positioned in a manner that causes the magnetic field produced to be in a vertical plane.
The other two coils, also connected in series, are positioned so that their magnetic field is in a horizontal
plane. The coils (N-S) which produce a horizontal field are called the VERTICAL DEFLECTION COILS
and the coils (E-W) which produce a vertical field are called the HORIZONTAL DEFLECTION COILS.
This may be more clearly understood if you recall that an electron beam will be deflected at right angles
to a deflecting field. The deflection coils are illustrated in view A of figure 3-7. View B shows the N-S
windings in schematic form.
Figure 3-7.—Deflection yoke.
