How Johnson measured Johnson-Nyquist noise?
Here's Johnson's original 1928 paper. May give you some clue :-)

Note that there are two papers - one by Johnson and the immediately following one by Nyquist. Both are from Physical Review July 1928 with Johnson's paper immediately followed by Nyquist's. J's title is "Thermal agitation of electricity in conductors" and N's is "Thermal agitation of electric charge in conductors".  

Johnson:


https://facultystaff.richmond.edu/~olipan/Johnson.pdf


Nyquist:


http://www.physik.uni-augsburg.de/theo1/hanggi/History/Nyquist.pdf


Richmond.edu link below has a number of interesting papers. 



Useful "modern" comment

http://www.claysturner.com/dsp/Johnson-Nyquist%20Noise.pdf

Related:

Various papers including Johnson's from above:


https://facultystaff.richmond.edu/~olipan/teaching.htm


http://www.claysturner.com/dsp/Johnson-Nyquist%20Noise.pdf

http://kiwitobes.com/wiki/Thermal_noise.html

http://uk.ask.com/wiki/Johnson%E2%80%93Nyquist_noise

http://en.wikipedia.org/wiki/Talk%3AJohnson-Nyquist_noise

http://en.wikipedia.org/wiki/Johnson-Nyquist_noise

http://ocw.mit.edu/courses/physics/8-13-14-experimental-physics-i-ii-junior-lab-fall-2007-spring-2008/labs/lab13/