Re: [Phys-l] x-ray machine

[Brian Whatcott <betwys1@sbcglobal.net>]

Anthony Lapinski wrote:
> I saw a new dentist a few weeks ago. I could easily see the x-ray from the
> chair, and wrote down some information. I often use "real"
> devices/appliances in my classes when discussing electricity, Ohm's Law,
> etc.
>
> x-ray machine:
> 60 kV
> 7 mA
>
> Using Ohm's Law, we get:
>
> R = (60,000 V)/(0.007 A) = 8.57 x 106 ohms
>
> I thought this result was interesting -- either very high or Ohm's Law
> does not apply well in this case. Any ideas?
>   
Vacuum tubes ("thermionic valves") were and are rather high impedance 
devices,
and the anode resistance can be varied by means of a varying voltage on 
a control grid.
There is a measure of this control ratio called mutual conductance: it is
the change of anode current / the change of grid volts.

The characteristic curve of anode current versus voltage can be rather 
flat, once the knee is passed: this means that a variation in anode 
voltage has next to no effect on anode current - particularly so where 
there are one or more extra electrodes masking the control grid from the 
anode.

Since most vacuum tube applications were replaced by the far more energy 
efficient bipolar transistor, there was no great impetus to mimic vacuum 
tubes with a later development: the insulated gate metal-oxide silicon 
field effect transistor where this concept of a control voltage 
controlling an output current comes once more into play.
The bipolar transistor modulates its collector current by means of a 
base current,
and the impedances are by contrast, much lower.

Brian W