How Mr. Johnson, the one after which Johnson–Nyquist noise was named measured/discovered this phenomena? Details about equipment and method are most welcome.
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".
Richmond.edu link below has a number of interesting papers.
Useful "modern" comment
Various papers including Johnson's from above: