About
historic and modern machines for the generation of static electricity
Albert G. Ingalls, April 1955 and
C.L. Stong, October 1956
NEAR THE CLOSE OF THE 17th century Otto von Guericke,
a citizen physicist of Magdeburg, invented the world's first electrical
machine-an electrostatic generator. He wrote down these instructions for
building one:
Secure one of the glass globes which are called phials,
about the size of a youngster's head; fill it with sulfur, ground in a
mortar and melted by the application of a flame. After it refreezes, break
the phial, take out the sulfur globe and keep it in a dry place, not a
moist one. Perforate it with a hole so that it can spin upon an iron axle.
Thus the globe is prepared.
To demonstrate the power developed by this globe,
place it with its axis on two supports in the machine-a hand's breadth
above the baseboard-and spread under it various sorts of fragments such
as bits of leaves, gold dust, silver filings, snips of paper, hairs, shavings,
etc. Apply a dry hand to the globe so that it is stroked or grazed two
or three times or more. Now it attracts the fragments and, as it turns
on its axis, carries them around with it.
When a feather is in contact with the globe, and afterwards
in the air, it puffs itself out and displays a sort of vivacity . . .
and if someone places a lighted candle on the table and brings the feather
to within a hand's breadth of the flame, the feather regularly darts back
suddenly to the globe and, as it were, seeks sanctuary there.
After describing numerous other experiments, in some
of which the globe produced light and sound, von Guericke concluded: "Now
many other mysterious facts which are displayed by this globe I shall
pass by without mention. Nature often presents in very commonplace things
marvelous wonders which are not discerned except by those who through
insight and innate curiosity consult the oracle of experimentation."
Ten generations of experimenters have consulted the oracle
since von Guericke's day. So fathomless are the mysteries of his sulfur
ball that its "marvelous wonders" continue to charm and sometimes
baffle experimenters, whether they twiddle the controls of a Van de Graaff
accelerator or merely stroke the hairs on a cat's back. But electrostatics
has remained chiefly a curiosity. Most people are acquainted with it only
in the form of the crackling shock you get when you touch a metal doorknob
after walking across a thick carpet in winter. Those who go in for amateur
radio grumble that "static is something you cuss, not study!"
Static electricity has never become economically important,
probably because nature has been more generous in supplying effective
conducting substances and magnetic materials than it has in providing
good insulators. Our electrical technology is based on electromagnetic
devices; our electrical power is generated by magnets and moving conductors.
Electrostatic machines have been harnessed for only a few specialties,
such as generating high voltage for laboratory experiments and producing
high-energy X-rays and sterilizing drugs in medicine.
Some engineers once believed that electrostatics had
big technological possibilities. One of them was John G. Trump, professor
of electrical engineering at the Massachusetts Institute of Technology.
He pointed out that the forces resulting from the presence of electric
charge are the most direct and powerful in nature. He and his colleagues
at M.I.T. conducted experiments in the 1950s aimed at producing an electrostatic
power generator could compete with the electromagnetic generator.
Professor Trump's idea can be illustrated by considering
two metallic plates, 100 square inches in area, facing each other and
separated by an insulator.
If a voltage amounting to an electric field of 300 volts
per centimeter is applied between them, the plates will be attracted to
each other with a force of one 2,000th of a pound. Increase the field
to 80,000 volts per centimeter and the attraction becomes half a pound.
Now immerse the plates in a high vacuum-a good insulator, though one difficult
to maintain-and increase the field to three million volts per centimeter.
The force of attraction jumps to 5,700 pounds! "Force of this order,"
declared Trump, "has more than passing interest for power engineers."
Trump and his associates tried to develop a practicable
system of vacuum insulation that would make possible a field intensity
of millions of volts per centimeter in a large machine. They never succeeded.
But if they had, an electrostatic power generator could become a reality
one day. The working parts of such a machine would look like an oversized
variable capacitor with intermeshing leaves. Only the rotor would move.
The machine could be constructed of light metal. A generator of this type
about the size of a small bedroom and weighing only a hundred kg (a few
hundred pounds) could deliver 7,500 kilowatts of power. Its efficiency
would be impressively higher than that of an electromagnetic generator.
The history of electrostatics began with the discovery
of Thales of Miletus that rubbed amber attracted other objects. Through
the centuries experimenters like von Guericke explored the "marvelous
wonders" of static electricity with a succession of ingenious machines
which amateurs may enjoy building.
One of the most important historically was Alessandro
Volta's elettroforo perpetuo, now known as the electrophorus
[see Fig. 1]. Volta wrote to Joseph Priestley of the Royal Society on
June 10th, 1775: "I hereby draw your attention to a body that, after
being electrified by a single brief rubbing, not only does not lose its
electricity but retains obstinately the indications of its active force
in spite of being touched repeatedly any number of times." The electrophorus
consists of two working parts: (1) a rectangular block of insulating material
such as Lucite (trademark of DuPont) or, preferably, polyethylene, and
(2) a metal disk fitted with an insulating handle. When the Lucite is
wiped with a woolen cloth, electrons are removed from the cloth and deposited
on the Lucite-in much the same way that a dirty rag smears a clean sheet
of glass. Early experimenters (who used sealing wax, hard rubber or other
resinous substances instead of plastic) said that the insulator thus stroked
had been "electrified by friction." They failed to observe that
the cloth took on an equal and opposite charge. When electrons rub off
from the wool to the Lucite (because, as it happens, some of the atoms
in wool hold electrons less tightly than those in Lucite do), the atoms
that have lost electrons become positively charged ions. Accordingly the
surface of the wool is peppered with tiny areas of positive charge while
the Lucite has similar areas of negative charge. The charges are static-bound
at the places where they are deposited-because in an insulator electrons
cannot move freely about in the substance.
Ever since Benjamin Franklin named the "positive"
and "negative" ends of the electric field, the field has been
thought of as originating at the positive end. He might have prevented
confusion if he had assigned the names the other way around, because we
now have to say that the current "flows" from positive to negative,
although actually the electrons flow from negative to positive!
The field between a pair of opposite charges is pictured
as a pattern of curved lines, called field lines, that radiate away from
positively charged objects, containing matter that is deficient in electrons,
and converge on things that are negatively charged and so contain an excess
of electrons. The direction of these lines show at each point the direction
that a tiny positive charge would be pushed if released there. What's
more, the density of these lines at a given point is proportional to the
strength of the force at that point. It turns out that these pictures
have tremendous mathematical force. They represent the electric field
and are quite useful for predicting how groups of charges will behave
when brought into close contact.
Von Guericke demonstrated that an electrified insulator
would transmit some of its charge to one that is electrically neutral.
We now know that it accomplishes this by sharing its excess electrons
with the uncharged body at points where the two touch. Von Guericke also
showed that a body could become temporarily electrified merely by entering
a field of charge, without touching the charged surface. He wrote: "If
a linen thread supported from above is brought near the globe and you
try to touch it with your finger or any other body, the thread moves away
and it is difficult to bring the finger near the thread."
This was an important discovery. It demonstrated that
charging by induction does not exhaust the charge on the body initially
electrified, and also that charges of like sign repel each other. Charging
by contact involves the sharing of electrons between two bodies, and each
contact diminishes the number remaining on the charging objects. Charging
by induction, in contrast, makes no demand on the free electrons but only
on the field set up by them. The field causes the electrons of the uncharged
substance to shift slightly from their normal orbits so that the center
of their orbit is no longer the center of their atom. In an insulator,
these electrons are bound to their atoms and so cannot move too far. Still
when their orbits distort the positively charged nuclei are no longer
perfectly canceled out by their electrons. This results in a slight excess
of positive charge on the side the electrons are pushed away from, and
a slight excess of negative charge on the other side. Since the surfaces
charges go away as soon as the external charge is removed, one says that
an external charge "induces" charge in the previously uncharged
insulator.
Something similar happens in a conductor, where the outer-most
electrons are free to wander away from their atoms. However, induction
can give a conductor a permanent charge, in the sense that the charge
will remain on the conductor until it leaks off or is otherwise dissipated.
All modern electrostatic generators are designed on the principle of inductive
charging. And the charging process involves the transformation of mechanical
energy into electrical energy. The principle is nicely demonstrated by
the electrophorus.
Here's how a conductor can be charged by induction. When
the metal disk is placed over the charged Lucite, the Lucite's negative
field opposes that of the electrons near the lower surface of the disk.
Electrons are free to move in a conductor, and so this force drives some
electrons off of their atoms and towards the upper surface of the disk.
Consequently the lower surface becomes positively charged and the upper
surface negatively charged. If, while the top of the metal is thus charged,
you touch the top with your finger, the excess electrons will flow into
your body-where, electrically, things are not so crowded. Now if you take
your finger away and then lift the metal disk by the insulating handle,
a net positive charge is trapped in the disk.
This method of charging removes no electrons from the
Lucite, nor does it draw on the Lucite's energy. Yet the metal disk is
now energized with positive electricity (many protons have been denuded
of their neutralizing electrons and hence their positive fields extend
out into surrounding space). The disk will now attract other bodies just
as the charged Lucite does. Moreover, a spark will jump between the charged
disk and your finger-which you can easily observe in a darkened room.
The energy expended in the spark came from your muscles when you lifted
the metal from the Lucite. The spark was created by electrons colliding
with molecules of air in their headlong rush from your body back into
the disk.
A more effective arrangement for generating electrostatic
energy by induction uses two Leyden jars [see Fig. 2]. One jar, A, has
a tiny positive charge. When its positively charged terminal is brought
close to a brass ball, A', electrons in the ball are attracted to the
side of the ball nearest the terminal. Similarly the terminal on another
jar, B, with a small negative charge, drives away electrons in the ball
B', making the near surface positive. If the two balls are now connected
by a metallic rod [middle drawing], electrons in B' (repelled by the field
of B and attracted by that of A) will flow to A'. Removal of the rod traps
the charges-just as the removal of your finger trapped those in the disk
of the electrophorus. Now suppose we change the positions of the balls,
moving A' toward B and B' toward A [bottom drawing]. To do this we must
expend work, because A', for instance, is repelled by B and attracted
by A. This work is transformed and stored as potential electrical energy
as soon as we touch A' to B and B' to A. The excess electrons in A' flow
into B, and A similarly acquires an increase of positive charge. The cycle
can be repeated indefinitely. In theory the amount of energy stored in
the jars (capacitors) can be increased without limit. In practice, the
storage is limited by the fact that electrons leak away more and more
rapidly as the charge increases.
Various induction machines have been designed for performing
the sequence of operations automatically and with considerable speed.
In these machines the "carriers" take the form of thin metallic
sheets instead of balls, and the capacitors also are metal sheets, called
field plates.
An early form of the machine, patented in 1860 by C.
F. Varley of England, is easy
to construct [see Fig. 3]. It consists of a pair of field plates cemented
to a square slab of Lucite surmounted by a rotating disk of Lucite to
which six or more sectors of aluminum foil are cemented. Two brushes (of
tinsel) momentarily connect opposite sectors, the carriers, with their
respective field plates as each carrier enters the region of its plate.
A similar pair of brushes again makes contact with opposing pairs of carriers
as they move from the region of the field plates. A pair of "corona"
combs-quarter-inch metal rods fitted with steel phonograph needles spaced
half an inch apart-graze the carriers at positions intermediate between
the two sets of brushes. The machine's electrical output flows from the
combs to a pair of spheres an inch or so in diameter that comprise a spark
gap.
The lower left diagram illustrates the action. Assume
a charge on the field plates [outer solid segments]. Electrons flow into
the carrier at the left, leaving the right-hand carrier with a positive
charge. Work is now expended in moving the carriers "up the potential
hill" to the opposite field plates. Here they make contact with the
brushes and part of their newly acquired energy flows into the field plates;
electrons enter the field plate [top of drawing] from the negatively charged
carrier, while the opposite carrier withdraws electrons from the lower
plate. The succeeding action of all carriers is the same. After a short
period of operation the combs reach ionizing potential, and energy flows
from the carriers to the gap, where vigorous sparking occurs.
The machine is not very efficient. This can be demonstrated
by observing its operation in a dark room. The rotating carriers appear
as a blurred disk of phosphorescence in colors ranging from greenish-blue
through violet, while the field plates are outlined sharply in purple.
Corona discharge at the combs is quite brilliant. This display means that
electrons are streaming from the thin, sharp edges of the foil and the
points of the comb carrying negative charge and into those parts carrying
a positive charge. Considered as an electrical "pump," the machine
is leaky and thus wastes energy.
The corona effect is explained by the geometry of the
machine's conducting parts. The electric from a point charge field radiates
into space uniformly in all directions. If the charge is enclosed by a
conductor, the lines of force always emerge perpendicular to its surface.
In the case of a spherical conductor (in effect, an enlarged point) the
lines are, therefore, distributed uniformly over the surface. When the
sphere is distorted to an egg shape, however, the lines bunch up at the
little end and thin out at the big end-because they must emerge everywhere
at a right angle to the surface. Crowding at the little end becomes more
pronounced as the radius of the "point" is made smaller. This
is another way of saying that the intensity of the field, or the potential
gradient, increases inversely with respect to the radius of the conductor;
in theory it would approach infinity at the point of a perfect needle.
Even in practice, finely made points can concentrate fields of astonishing
intensity. The exquisite needles used in field-emission microscopes create
field intensities of 750 million volts per inch in the immediate vicinity
of the point-although the instrument operates from a power supply of only
5,000 volts! [Those of you who think that field emission microscope are
something new should check out "A New Microscope" by Erwin W.
Müller, Scientific American, May 1952.] At this field intensity
electrons are literally ripped from the metal point and ejected radially
into space. Gas, if present in the tube, becomes heavily ionized. The
collecting combs of the Varley machine similarly ionize adjacent air,
negative charges being carried by dislodged electrons and positive charges
by the ions.
In the early years of the 20th Century the
Wimshurst generator, similar in basic principle to the Varley but carrying
one or more pairs of disks that rotate in opposite directions, was a favored
source of power for X-ray machines and other devices requiring relatively
small amounts of current at high voltage. The largest machines carried
as many as 12 pairs of disks seven feet in diameter and delivered potentials
on the order of 200,000 volts.
The Wimshurst and other electrostatic generators of this
era could not reach the million-volt range. By 1920 they had been largely
replaced by electromagnetic induction coils and transformers as sources
of high-voltage power.
The modern era of electrostatics began in 1929. In that
year Robert J. Van de Graaff, a young Rhodes scholar from Oxford University
who was working at Princeton as a National Research Fellow, invented the
electrostatic belt generator that is now known around the world by his
name. He was interested in developing a steady constant-potential voltage
with which to accelerate atomic particles to bombard nuclei in order to
obtain evidence of their internal structure. Today the Van de Graaff accelerator
can be found in nearly every large nuclear laboratory in the world; it
is the workhorse for precision research in this field. The accelerator
has attained a particle energy of more than eight million volts. In smaller
sizes the machine has found a wide variety of applications, particularly
as the power source for the high-voltage X-ray treatment of disease.
One of the nicest features of the Van de Graaff generator
is its relative simplicity and low cost. Robert W. Cloud of the High Voltage
Research Laboratory at M.I.T. has designed a small version as a special
construction project for amateurs [see Fig. 4]. Its action is as simple
as its design. A motor developing 3,000 revolutions per minute is housed
in a coffee can. It drives a gum-rubber belt that passes over an insulated
pulley inside the upper terminal. Spray screens, counterparts of the Varley
machine's collecting combs, are situated close to the surface of the belt
at each end of its run, and each connects with the respective terminal.
As the machine goes into operation, frictional contact removes electrons
from the belt at the driving end and deposits them on a plastic pulley.
Positive charges resulting at the sites on the belt that have thus lost
electrons are then carried by the belt to a metal pulley at the other
end above. Electrons flow from the metal pulley onto the electron-deficient
belt. As the machine continues to run, heavy charges build up on both
pulleys.
After a few seconds or minutes, depending on the humidity
of the air, the field originating at the pulleys reaches ionizing intensity
in the vicinity of the spray screens. Electrons are then withdrawn from
the upper terminal and sprayed on the belt at the beginning of its downward
run. Similarly electrons en route down the belt come within the region
of ionization at the lower spray screen and flow by way of its supporting
bracket into the lower terminal. Through this pumping action the belt
continuously exhausts electrons from the upper terminal and discharges
them into the earth through the lower one. This leaves the upper terminal
with a net positive charge that, because of mutual repulsion of the positive
"holes," distributes itself uniformly over the terminal's outer
surface. Accordingly the inner surface carries no charge. In theory, voltage
across the upper and lower terminals increases without limit. As in the
case of the Varley and Wimshurst machines, however, the charge is limited
by the quality of the insulation. At about 100,000 volts charge leaks
as corona from the upper terminal and as conduction current down the insulating
column at a rate equal to the two microamperes which the belt is able
to carry into the terminal. Although 100,000 volts is an impressive value,
the machine creates no shock hazard because the capacity of the upper
terminal to store charge is small.
If a well-rounded object is brought within a few centimeters
(an inch) or so of the high-voltage terminal, a spark will jump. In this
type of discharge the air is rapidly changed from a good insulator to
a conductor and the spark completely discharges the terminal. Reduction
of the terminal voltage permits the air to regain its insulating strength,
and the terminal is recharged by the belt. If an object with sharp edges
is brought near the terminal, it steadily drains charge by corona and
decreases the potential. Such a device, with adjustable spacing, is often
used in Van de Graaff generators to maintain a constant terminal potential.
A continuous source of high direct current voltage invites
endless experiments. One of the most amusing is the "jumping ball"
demonstration. A half dozen small balls made of pith or other light material
are given a conducting surface of soot or graphite. They are placed in
a cage, which may be made of a strip of transparent plastic rolled into
a cylinder and capped with tops from peanut-butter jars. The caps are
connected with the terminals of the Van de Graaff. As the machine goes
into operation, the electrostatic field from-the upper cap attracts the
balls. They hop up to it, deliver their load of electrons, fall back and
repeat the cycle as long as power is supplied.
Support a sewing needle by an insulator and connect it
to the machine's upper terminal. Molecules of ionized air will rush from
the point as though they were streaming from a jet under pressure. They
easily blow out a match or candle. This electric wind can be made to drive
a simple motor. Cut a swastika, with sharply pointed tips, from aluminum
foil and indent its center with the pointed end of a pencil. Pivot the
indentation on the point of a pin that has been thrust up through a supporting
base of cardboard. The swastika will then be free to rotate on the pinpoint.
It will do so vigorously if the pin is connected with the high-voltage
terminal of the Van de Graaff. Ionized air streaming from the four points
sets up the reactive force of a jet engine.
The power capacity of the Van de Graaff is enough to
charge a person to about 50,000 volts. This is 20,000 volts above the
ionizing point of air at atmospheric pressure. It is also enough to make
the experimenter's hair stand on end. To demonstrate this effect, stand
in or on a large glass bowl or a wooden platform supported by four square
milk bottles. Touch the high-voltage terminal. After a few seconds your
hair will slowly rise. Incidentally, the body adds capacity to the terminal,
and so a somewhat larger charge than normal accumulates. When you step
down or touch a grounded object, you will experience a slightly painful
shock-but it is not dangerous to a person in normal health.
Fluorescent lamps will light up brilliantly where they
are touched to the high-voltage terminal. If the room is not too brightly
lighted, filament-type lamps also will glow in various colors depending
on the kind of gas they contain. You can even manufacture a miniature
aurora borealis by boiling water in a thin flask until the air is displaced
by steam and then stoppering it immediately. After the steam has condensed,
the rarefied air inside will glow greenish and pink when the flask is
brought in contact with the Van de Graaff.
Those who can blow glass and exhaust it-or who can induce
a local manufacturer of advertising signs of the glow tube type to do
so for them-may want to try their hand at assembling and operating a linear
accelerator and related apparatus used for nuclear research. Such projects
are on a par with amateur-built cyclotrons and, like marriage, are not
to be entered upon lightly. They are, nevertheless, well within reach
of amateur resources, particularly for groups.
To power such an apparatus you will require a larger
version of the Van de Graaff [Fig. 5]. It differs from the low-power design
in a number of subtle, though important, particulars. The spray points
for charging the upward run of the belt are supplied by a potential of
5,000 to 10,000 volts from a transformer-rectifier combination. The high-power
machines employ metal pulleys at both ends of the belt, the upper one
being insulated from the high-voltage terminal.
Charge is sprayed onto the belt as it passes through
the corona between the lower points and the grounded driving pulley. A
similar set of points, located just inside the upper terminal, removes
charge from the upward belt run and conducts it to the upper pulley. After
a short period of operation the upper pulley acquires a high charge and
current flows to the upper terminal through a current-regulating resistor.
This circuit may also include a corona gap near the inner surface of the
terminal. A second set of spray points (charging rod), connected directly
to the high-voltage terminal, is situated at the top of the pulley. The
difference in potential between the upper pulley (made "live"
by the voltage drop across the current regulating resistor and corona
gap) and the high-voltage terminal causes these points to spray a charge
of opposite sign onto the downward run of the belt. The value of the current-regulating
resistor is chosen so that both sides of the belt work equally. The value
of the current-regulating resistor can be computed roughly by Ohm's law.
Belts for high-power machines are usually made of rubberized fabric and
run at speeds of 1,200 to 1,800 meters (4,000 to 6,000 feet) per minute.
The capacity of the upper terminal to store charge varies
with its size. Its ability to hold charge varies with shape. The ideal
terminal would be spherical. Unfortunately this ideal cannot be realized
because provision must be made for the entry of the belt. The shape must
be such that the intensity of the field at the high-voltage terminal is
always less than the value at which spark or corona discharge occurs.
Hence the aperture of the terminal must have re-entrant edges and the
facing sides of the upper and lower terminals should be identical. Such
terminals are commonly made of aluminum spinnings.
Large Van de Graaffs in the million-volt range, intended
for scientific and industrial purposes, are now nearly all mounted within
a steel tank containing Freon (trademark of DuPont) , carbon dioxide or
a similar gas at many atmospheres of pressure. The high pressure serves
to increase the voltage-insulating ability of the gas many-fold and thus
increases both the voltage and current capacity of the machine.
When Van de Graaff machines are designed for potentials
above 200,000 volts, the distribution of charge along the insulating column
(and even along the belt runs) becomes important. The columns of air-insulated
machines using belts more than four inches wide should be fitted with
equipotential rings spaced along the insulating column at intervals of
about two inches.
Anyone undertaking the construction of a high-power Van
de Graaff should remember that he is building no toy. These potentially
lethal machines can reveal "marvelous wonders" beyond von Guericke's
most inspired imagining, but, unlike his sulfur ball, they pack the wallop
of lightning!
F J. Hedley, of Lancaster in England, submitted an interesting
version of the Van de Graaff to this department. His machine demonstrates
that a resourceful amateur can sometimes improve on the performance of
a design by substituting his own materials. Hedley writes the following
report on his machine.
"I have now made two Van de Graaffs. One, shown
here, performs quite satisfactorily, giving bright intermittent sparks
about four inches long.
"The machine is supported by a housing which encloses
the motor and lower comb assembly. The housing consists of 1/16-inch sheet
steel welded to form a rectangular box measuring six inches high, six
inches wide and eleven inches long, the base being of 3/16-inch steel.
A detachable cover at one end provides access to the motor. One side can
also be removed for exposing the pulley, comb assembly and belt.
"The belt is powered by a series-wound motor which
runs under full load at a speed of 6,000 revolutions per minute. This
is fitted with a pulley machined from polyethylene, one and a quarter
inches in diameter and one and three quarters inches long. It is crowned
slightly and makes a driven fit with the shaft. A wooden pulley of the
same size with a jacket of polyethylene would work equally well. The jacket
could be covered with a layer of polyethylene friction tape of the kind
now sold by dealers in electrical supplies, or it could be made of a cylindrical
section cut from one of the small polyethylene bottles in which cosmetic
preparations are commonly sold.
"The lower comb assembly consists of a bracket made
of 1/16-inch sheet copper, an inch and five eighths wide, to which six
tufts of number 86 copper wire are soldered at one end. The wires are
spread out to form an even row and clipped straight at the ends [see drawing
in Fig. 7]. The brush assembly is bolted to the base plate and adjusted
to bring the wire comb slightly above the center of the pulley.
"A switch for operating the motor and a small neon
bulb are mounted in the fixed side of the housing. The lamp glows when
the machine is operating properly. Originally I mounted it on top of the
housing but lost several bulbs when they were perforated by high-voltage
sparks!
"A tube of clear plastic, three inches in diameter
and 18 inches long, which makes a push fit with a polished pipe flange
bolted to the housing, supports the upper terminal. The inner threads
are filed from the flange prior to assembly. The upper end of the tubing
is beveled, and two holes are drilled on the diameter to receive a pair
of ball bearings. A X-inch shaft turns in these and supports a crowned
brass pulley that matches the size of the lower pulley. The pulley makes
a sliding fit with the shaft and is anchored with a setscrew. This arrangement
provides for easy disassembly when changing the belt.
"The high-voltage terminal consists of a nine-inch
spherical copper float of the type commonly used in industrial plumbing.
It is split on the diameter by means of a thin hacksaw blade. A flat,
narrow ring is soldered to the inner edge of the lower half, on which
the upper half makes a snug fit and clean joint. A two-inch hole is cut
in the center of the lower hemisphere and the metal is worked into a doughnut
shape by means of a rawhide hammer and rounded anvil. A cylindrical collar
is then soldered to the inner face of the re-entrant edge for an easy
fit with the plastic tubing. The upper collecting comb is bolted to a
lug soldered to the inner surface of the lower hemisphere. A kink in the
bracket provides for final adjustment relative to the center of the upper
pulley.
"A disk of 1/4-inch plastic, six inches in diameter,
serves as a platform for supporting the upper terminal. A hole is cut
in the center for admitting the plastic tube, to which the disk is fastened
by one of the quick-drying plastic cements. In addition to supporting
the terminal, the disk minimizes corona discharge from the bottom of the
terminal. A discharge gap is provided at the side of the machine, as shown
in the drawings.
"These machines are sensitive to the high humidity
prevalent in England, and for
reliable operation I found it necessary to install a 50-watt heating element
inside the motor housing. My next project will be a Van de Graaff of the
type in which the charge is sprayed onto the belt at the lower pulley
by a high-voltage source. I should appreciate any constructional tips
that amateurs who have built such machines would care to pass along."
John G. Trump, Electrostatic Sources of Electric Power,
Electrical Engineering 66, 532-534 (June 1947).
J. G. Trump, Electrostatic Sources of Ionizing Energy,
Transactions of the American Institute of Electrical Engineers
70, Part 1, 1021-1027 (1951).  |
