[time-nuts] Garage built Rubidium standard

Brucekareen at aol.com Brucekareen at aol.com
Sat Jan 15 18:52:24 UTC 2011

The "Garage-Built Rubidium Standard" discussion brought to mind an  
setup in a physics lab at U. C. Berkeley years ago.  It was one of a  
number of 
elective experiments for the laboratory section of a Modern Physics  
The objective was probably to demonstrate Zeeman line splitting.  I  
much of the equipment was surplus from the nearby Radiation  Laboratory.
The setup was shrouded by heavy black theater curtains that hung from the  
ceiling to 
darken the experimental area.  There were two, about  one-inch-diameter, 
filled glass spheres, each surrounded by  Helmholtz coils.  The first also 
had a toroidial 
resistance heater coil  controlled by a Variac.  It served as the light 
source when 
energized by RF from the Helmholtz coil.  The temperature had to be in  the 
range in order for the bulb to luminance.   
The second bulb was the reaction sphere.  Its Helmholtz coil was fed  with 
a trapezoidal 
waveform from a sweep generator along with direct current from a powers  
supply.  The 
Helmholtz coil also received an RF signal of about 50 MHz from a  General 
Radio unit oscillator 
(possibly through an amplifier).  A rather expensive-looking, ¼-inch  
thick, roughly 2-inch 
by 2-inch filter or diffraction grating (that was normally carefully kept  
is in a heavy-wool 
envelope) was placed in a holder between the spheres.  There were  probably 
some other 
optical elements too, but it has been too long to remember.  
On the far side of the reaction sphere was some sort of photodetector that  
fed an old 
Dumont oscilloscope with the beam being swept horizontally from the sweep  
If you managed to get the pip on the scope (not many student-pairs  
succeeded) the rest 
was calculation.  One of the required calculations was the force of  
gravity acting on the 
experiment.  This experiment was  notable as an instructive exercise in 
getting old, 
recalcitrant test instruments to all function at the same time, more so  
than the 
theoretical physics.  If one could acquire the rubidium –filled  spheres 
and filter, a 
working rubidium standard could probably be constructed in the  garage.
Bruce, KG6OJI

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