Re: Megavolt at $90 (fwd)

[Steven Roys <sroys@xxxxxxxx>]

---------- Forwarded message ----------
Date: Mon, 15 Mar 2004 06:55:33 EST
From: Lyonelb@xxxxxxx
To: hvlist@xxxxxxxxxx
Subject: Re: Megavolt at $90 (fwd)


15/03/04 4:01:22, hvlist@xxxxxxxxxx :

<<The first paper about the VDG generator was a quick note at the Physical
Review, vol 38, p. 1919, 1931.>>

The text of IEEE Spectrum contains some errors ( <<The belt is of insulating
material, but has at intervals carriers of conducting material >>).

Megavolt at $90

The new 35-M/ V Vivitron accelerator being tested in Strasbourg, France,
-1990-,  marks the latest step in meeting a technological challenge first taken up
by a young Rhodes Scholar back in 1925.

Robert J. Van de Graaff was 24 when he went to Oxford in 1925. Soon
afterward, he encountered a question in his physics text that remained in my mind as a
sort of challenge, he recalled later: why was it impossible to build an
electrostatic machine with a large power output?

A memorandum he produced in 1933 reveals that his first major insight was
that he could accelerate ions and electrons to -enormous- energies by generating
a high voltage in a vacuum. At the same time, he recognized that direct
current would be preferable to the more usual sources which have alternating,
rippling or impulsive characteristics.

Actual experimental work had to wait until September 1929, when the new Ph.D.
arrived at Princeton University in New Jersey to work under physicist Karl T.
Compton on a National Research Council (NRC) fellowship. Only a month later,
Van de Graaff was able to show Compton the first model of his electrostatic
generator. Its operating principle was explained formally the following March in
a report that, as a fellow, Van de Graaff had to write for the NRC.

Referring to a drawing , he wrote: A motor driven pulley P drives by means of
the belt B a second pulley Q, which runs freely. The belt is of insulating
material, but has at intervals carriers of conducting material . As these
carriers pass under the inductor I, maintained at a considerable negative potential
by an auxiliary Whimshurst machine, there is induced on each carrier a bound
positive charge, while the free negative charge escapes to earth through the
grounded metal pulley P. Thus the carriers move away toward the anode A carrying
a positive charge, which they retain until they make contact with the metal
pulley Q, situated in the interior of the anode and connected with it. At this
contact the carrier gives up its entire charge to the surrounding anode and
then returns uncharged to the first pulley P. In this way positive charge is
continually brought up by the moving belt to the anode, so that its potential
rises steadily, either until a constant potential is reached at which the leakage
from the anode becomes equal to the input current, or until a spark passes.

 The electrostatic machine was easy to build. The first model, dubbed the tin
can generator, had a belt made of silk ribbon bought at the local
five-and-dime (where, the story goes, Van de Graff alarmed the salesgirl by testing the
ribbon's flammability). Operated in the open air, it reached a potential of 80
000 volts.

A few months later, Van de Graaff generated over  1 000 000 V between two
60-centimeter-diameter spheres on Pyrex columns 180 cm high. This version was
published Oct. 28, 1931, in a three-page "Disclosure of invention," signed by Van
de Graaff and two witnesses. The apparatus cost $90.

Van de Graaff's generator made headlines when it was demonstrated in New York
City on Nov. 10, 1931, before a dinner meeting of the newly formed American
Institute of Physics. "Atom Nucleus Seen Yielding To Science," declared The New
York Times in a front-page story, which reported that the Massachusetts
Institute of technology (MIT) in Cambridge was embarking on machines capable of
generating up to 15 million volts. Time magazine hailed the "$90 Lightning."
Compton called it "the most important development that has ever taken place in the
field of extremely high voltages."

Van de Graaff followed Compton to MIT in November and devoted the rest of his
life to building more powerful and sophisticated electrostatic accelerators
for industrial, scientific, and medical applications. He applied for a patent
on Dec, 16, 1931, and assigned it to MIT in 1933 under an agreement that gave
him 20 percent of the net income up to a maximum of $400 000.

On March 18, 1935, MIT vice president Vannevar Bush wrote Van de Graaff to
inform him that patent 1991 236 had been issued on Feb. 12. "It is a good
looking patent, and I hope that our dreams in connection with it may some day come
true," Bush added. In the light of the frontiers of nuclear research and cancer
treatment the invention opened up in the ensuing years, it is likely those
dreams were realized.

(Michael F. Wolff Contributing Editor - IEEE Spectrum  july 1990)