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On 2/1/17 10:10 AM, Brian Hall wrote:
One of Nikola Tesla's late night "party trick" demonstrations, in the
era where people such as Mark Twain would come to his New York City
lab, was to light up the room bright as day with no obvious source.
(Tesla: Man out of Time by Margaret Cheney)
almost certainly basically an RF excited fluorescent or glow lamp..
There's drawings of "metal buttons" in a evacuated envelope, presumably
with some low pressure gas to ionize. Think of a modern "plasma sphere"
.. the other approach is to use RF to heat the electrode to incandescence.
There are commercial systems today using microwave energy to excite
sodium vapor for lighting.
Bear in mind that era, the primary source of light was kerosene lamps,
gas lights, and candles. Life was dimmer at night back then.
(If I made an error in these estimations please let me know.) I am
curious if generating such light was by means of creating an
oscillator that resonated in the visible light spectrum.
An example of an oscillator that resonates in the visible light spectrum
is a laser. A gas laser has a resonant cavity formed by the two
mirrors. There's a lot of wavelengths between those two mirrors, so
there's actually a comb of frequencies coming out (e.g. if the
wavelength were 1 micron, and the mirrors were 100 mm apart, you could
have 100,000 wavelengths, or 100,001 wavelengths, or 100,002, etc.
There are techniques to enhance or suppress particular lines in the comb
(for instance, say you have a another resonator, it it has a different
set of resonances, and only one is common between the two...
Coupled resonators have many, many uses beyond tesla coils...
Given
visible light between 470 and 650 nm, let's pick 500 nm. The
frequency would be f=c/wavelength = (300x10^6 meters per
second)/(500x10^-9 meters)= 0.6x10^-15 = 600x10^12 Hz, or 600 THz.
That's an insanely high frequency! Consider 2.4 GHz from your
kitchen microwave or WiFi, 1 terahertz is of course 1000x greater
than 1 gigahertz...
With L and C components at that frequency, even with a 1 pF
capacitor, L = 1/((6.28^2)*((600x10^12)^2)*1x10^-12) = 70.4x10^-21 or
70.4 zeptohenries of inductance! That's three jumps smaller than pico
on the SI scale
https://ned.ipac.caltech.edu/level5/Units/frames.html
Either way, the inductance or capacitance would have to be incredibly
tiny. How would one create an oscillator that could run at 600 or so
THz, with either modern technology, or just what was available in
Tesla's day? I recall reading there are some airport security
scanner technologies operating in the low THz in development, but
don't recall those articles getting into detail about the components
or design of such an oscillator.
They're fairly exotic devices - passive THz units use either a bolometer
(sensing the energy directly by measuring the heat on a sensor) or some
form of superheterodyne receiver which relies on a harmonic mixer: e.g.
you drive the mixer at, say, 100 GHz, and the 11th harmonic at 1.1 THz
mixes with the input signal to produce a microwave IF, which you then
process conventionally.