need a 100K-Megaohm resistor (100 Giga-Ohm)

[Alex [Temple Boatanchors]]

At 03:42 PM 8/7/2006, you wrote:
>      A greater concern is the breakdown voltage. Many of the glass 
> sealed resistors are sealed with low pressure gas and have a low 
> break down voltage, the low pressure gas ionizes easily. Some have 
> atmospheric pressure dry gas and much higher break down voltage. 
> The glass ones are not as common today. High values are mostly 
> epoxie or silicone coated ones.  I have some silicone coated ones 
> that are 10 GOhms and I think they may make them in 100G values as 
> well, even these were special order and had to order a minimum 
> number of them for about $15 or $20 each.
>Another issue is getting a good CK5886 electrometer tube. These are 
>hard to come by nowadays and about half of them have a bit of gas in 
>them. Enough to make them useless due to ion currents. You may have 
>to get several before you find one with low enough grid current.
>
>73
>Bill wa4lav

Hi Bill,

The schematic of the project shows this resistor hooked up between 
the grid of the electrometer tube and ground. So basically the most 
voltage it should ever see is whatever potential ionization voltage 
is present in the measurement chamber. The chamber is "excited" by a 
67.5V battery [ -no chance of sourcing one of those ;-) ], so even if 
the chamber is accidentally shorted, the resistor will only see the 
battery voltage of 67.5V.

BTW, in case some of you are wondering, I decided to give this old 
Ionization Meter project a try just really for the fun of building 
it, and for the challenge of trying to source parts that where 
specified more than 30 years ago. Don't really need it to be 
accurate, or even to build it soon. So far I managed to get hold of 5 
electrometer tubes (NIB/NOS) and I hope at least one of them will 
still be in good shape (no gas, etc).

Thanks very much for your input and the information.
All the best
-Alex

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