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From "Electronic Fundamentals and Applications", John D. Ryder, 1964.

"Since the electron beam striking the screen is carrying negative charge and the screen is insulated by the glass, the removal of this charge is necessary; otherwise the potential of the screen would fall to such a negative value as to repel the beam. Fortunately, as the electrons strike the screen they not only cause the screen to give off light but also to emit other electrons. This effect, known as secondary emission (see Chapter 3), may result in an average of more than one secondary electron being emitted per electron in the beam. The secondary electrons are attracted to a graphite coating placed over the interior bulb walls and connected to the second anode. As a result, instead of the screen acquiring a negative potential, it may be a few more volts positive with respect to the second anode."