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Re: Undamped oscillations
Original poster: Ed Phillips <evp@xxxxxxxxxxx>
To do this the interval between each successive firing has to be
nearly identical. Once this is achieved, synchronization with the
transmitter's resonator is simply a matter of tuning. Tesla could
have used the alternator with, "a certain small number of [widely
spaced] poles" that was rotated at "an enormous speed" and generated
"sudden impulses which would produce the same effect as [a primary]
arc discharge," but there was a problem with the machine in that it,
"failed to give isochronous impulses."
The solution to this problem was an isochronous mechanical oscillator
that was used in place of the usual governor to control the inlet
valves of a 10 H.P. double compound reciprocating steam
engine. "Boiling water was employed to keep the temperature of the
air spring perfectly constant and the oscillations isochronous."
[NTAC, p. 39] The steam engine was attached to an electrical
generator. This machine was built in 1893. After destruction of the
35 S. 5th Ave. lab he built an improved apparatus that is mentioned
in an earlier post (Re: Wire Length, Jan, 16, 2007). This machine
had four vibrating parts instead of just one and furnished,
"isochronous currents of desired wave frequencies, phases, and
beats." [p. 45] This source of nearly constant frequency alternating
current was available to power a high-speed rotary break used in
conjunction with a high-voltage DC power supply.
"This is a form of break which I developed in working with
alternators. I recognized that it was of tremendous advantage to
break at the peak of the wave. If I used just an ordinary break, it
would make and break the current at low as well as high points of the wave.
Of this apparatus I had two forms; one in which I drove the break
right from the shaft of the dynamo and the other in which I DROVE IT
WITH AN ISOCHRONOUS MOTOR. . . ." [pp. 56-58]
I've read that before and found it most interesting. What a pity
it is that we don't have all his lab records from that time to know
exactly what he did, how he measured things, and what the results
were! I suspect that to him ISOCHRONOUS meant to a part in a
thousand at best, but plenty good enough for the purpose at
hand. The note from the one remaining Alexanderson alternator at
Grimeton, Sweden, appears to be stable (at least short term) to about
1 Hz out of 17.5 kHz but I haven't seen a description of the control
mechanism for the prime mover.