It was decided to experiment somewhat more.
Nuvistor electron tubes are great for playing with electrons. They provide space condition with no light so no photoelectric emission effects.
On part 1 we have seen that supplying a typical heater voltage makes the grid to cathode capacitance charged by many hundred millivolts. The grid negative with respect to cathode.
When the heater current is switched off the charge at the electrodes remains. Why should it not?
Should one assume for a moment that the space charge theory is wrong? Any theory has the potential to be wrong. Otherwise it would not had been a scientific theory [Karl Popper]
Any electron emitted from cathode cannot stay stationary. It has mass, velocity, momemtum. It staves in free space and gravity can be neglected. Inside a Nuvistor electrons should be emitted axial you around the cathode and move outwards until they strike the grid or the anode.
But let's first establish whether the grid gets charged negative from inside the electron tube and not from outside for example by the voltmeter 1GΩ path itself.
But even before heating up,the cathode lets supply a potential difference between the cathode and grid to see well the voltage can be measured by the 1GΩ input impedance voltmeter.
The same 7586 Nuvistor electron tube is used with flying leads connected to the Cinch Nuvistor socket.
Firstly an exhausted alkaline 1.5V battery was connected to YF-3200 digital voltmeter. Potential differences less than 300mV are aimed to as it is only at the most sensitive scale that this voltmeter has a Zin of 1 GΩ.
Tye battery measures -47mV. For some reason it's polarity has boome reverse over the years. When the voltmeter is connected the voltage slowly gets to -47nV. This should be due to the time constant of the voltmeter at such high input impedance. When the volt,enter is disconnected the reading is -40mV. It seems the test lead capacitance etc keeps the voltage. So in order to access what is happening with the electron tube the leads would have to be short circuited until zero volts is read before any meaningful measurement. In fact it was found preferable to switch off and on the voltmeter for reset.
Unfortunately it seems difficult to obtain meaningful measurements.
The digital voltmeter has an effective large capacitance. Therefore keeps the reading constant even after the leads are disconnected.
It was tried to charge externally the cathode to grid capacitance by -47mV, then 300mV by a difference battery.
After the battery was disconnected and then the reset voltmeter was connected all that could be read was -5mV to around -10mV.
It was found strange that in all case the grid was more negative to cathode. Even when the grid was charged with the positive terminal of the battery.
The voltmeter leads were swapped red to black, but still the reading was more negative on the grid.
Is some light still getting inside the electron tube?
Or is the cathode emoting even a few electrons at room temperature?
Perhaps more meaningful measurement can be made with an electrometer tube, FET electrometer, Keithley voltmeter etc.
Of course when heater voltage is applied the grid becomes many hundred millivolts negative and this can be read even with a 50KΩ input impedance voltmeter. But nothing can be said about whether electrons have entered the grid. It may be induced charge. For example nearby electrons repelling electrons inside the grid so that the grid terminal pin becomes negative.
No conclusion.
The grid reads always more negative even after short circuiting from cathode to grid. This again reminds photo emission or thermoemission from electrodes. Is the cathode emitting at room temperature?
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