Figure 2 is the end view of tube insides that have cylindrical construction. On the left there is a dot inside, and at the center of, a circle. The dot is labeled filament and the circle is labeled plate or anode. The circle is not touching the dot. On the right is a dot inside of three other concentric circles each one bigger than the last. The inner most dot is labeled heater. The next circle out which is touching the dot is labeled electrical insulator and thermal conductor. The next circle out which is touching the previous circle is labeled cathode. The outermost circle which is not touching the previous one is labeled plate or anode.