中和阴极灯丝电源的正极接地，使得整个中和阴极灯丝处于负电位；
Anode of power supply for neutralization cathode filament is connected to ground, so that filament is at negative potential.
左边的阴极被灯丝加热到高温，同时电子从阴极表面蒸发出来。
The cathode at the left is raised to a high temperature by the heater, and electrons evaporate from its surface.
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