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The
WISCONSIN
ENGINEER
VULUME 38, NO. 4
JANUARY, 1934
           The Ten Million
 THE gigantic electrostatic generator at the Massachu-
 Tsetts Institute of Technology may appear to be of
 a rather complex nature, but actually it operates on the
 simple principle of the familiar ice-pail experiment of
 Faraday. The outgrowth of Faraday's experiment is the
 Van de Graaff electrostatic generator shown on the cover.
 The photo gives an idea of the immense size of the up-
 right columns supporting spheres which serve as reservoirs
 for storing the electrical charges. The hollow spheres, which
 are fifteen feet in diameter and weigh 3000 lbs. each, are
 constructed of twelve sections of 1/4 aluminum alloy joined
 together by butt welds. After the welds were ground
 smooth, the entire surface was polished and waxed. The
 smooth surface and the large size of the spheres are neces-
 sary to reduce the corona discharge which is common at
 high potentials.
 The upright columns are six feet in diameter and more
 than twenty feet in height. The material used in the con-
 struction of the towers had to have high insulating proper-
 ties as well as the necessary mechanical strength to support
 the heavy spheres. Textolite, a material manufactured by
 binding layers of paper together with shellac under high
 pressure and temperature, was found to meet the specifica-
 tions very well. Molten ceresin was used to impregnate the
 exterior of the columns so that leakage due to surface mois-
ture could be minimized. The resistance of the impregnated
column is so high that only thirty microamperes pass to
ground when the sphere is charged to a potential of three
million volts. The design of the columns permits a spherical
terminal to be charged to over five million volts before a
flashover to ground occurs. These spherical terminals are
charged by two paper belts inside of the columns. A charge,
imparted to the belt at the foot of the column, is carried up
to the sphere where it is discharged. The paper belts are
four feet wide and seventeen mils thick and operate under
a tension of one-thousand pounds, the breaking load being
about six thousand pounds. Paper was used chiefly because
of its comparatively small cost. The insulating quality is
quite good, and no mechanical difficulties have presented
themselves even though the belt is run at a speed of over
sixty miles per hour.
l Volt Generator
   At the base of the column a ten horsepower motor drives
the belt which receives its charge from a "corona" wire
one inch away from the belt and charged to a potential of
twenty thousand volts. This high potential, which is ob-
tained from a kenotron rectifier set, causes a large number
of charges to move towards the belt where they are de-
posited and carried upwards. At the top, a similar wire
is enclosed in and attached to a metal shield (in effect, a
Faraday ice-pail) which collects the charges just before
they reach the upper pulley, and deposits them on the
sphere. On the other side of the pulley where the belt is
moving down, another "corona" wire is used to impart to
the belt the charges of undesirable polarity from the sphere.
At the lower end, the belt is discharged in a similar manner.
While one machine is storing up negative charges on one
sphere, the other is storing up positive electricity on the
other sphere, so that when each sphere is charged to five
million volts, the potential for which they were designed,
the potential existing between the two spheres is ten million
volts.
  It is obviously impossible to measure the voltage of the
generator by means of a voltmeter connected between the
two spheres. Modern radio technique has made it possible
to measure the strength of the electrostatic field, which is
proportional to the potential, by means of a sensitive vac-
uum tube amplifier. A revolving grounded shutter is em-
ployed to alternately shield and expose a terminal con-
nected to the input of the amplifier. The deflection of a
meter in the output circuit will thus be proportional to
the strength of the field, and may be calibrated to read the
potential of the spheres directly.
  Within each sphere a miniature laboratory has been con-
structed. In these laboratories, investigations will be carried
on to learn more about the atom, one of the many pur-
poses for which the generator was built. That high speed
electrical particles shall prove themselves an effective weapon
in breaking down the atom, is the hope of all these inter-
ested in the M. I. T. generator. Whether man will ever
be able to harness the tremendous energy that exists in the
atom, depends upon how much the physicist can learn
about the atom.
January, 1 934
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