Social Studies
Since the 17th century, scientists
have been fascinated with the topic of
static electricity. One of the first
famous static electricity machines
was created by Isaac Newton in 1690.
It used an insulated glass sphere and
a piece of cloth to create static electricity
through rubbing. Over the next century,
many famous scientists created similar
electrostatic generators and used them
in science, medicine, and entertainment.
The Wimshurst machine was the final
edition in a long line of devices that tried
to reliably produce static electricity.
James Wimshurst began inventing the
Wimshurst machine in 1880, only a year
after Edison’s light bulb, and finished in 1883.
The Wimshurst machine is an influence
machine, or an electrostatic generator,
meaning that it produces static electricity.
This, in its time, enthralled people,
as they were deeply interested in learning
how it functioned. In the late 1800's,
electricity was still a deep mystery, and
its components and principles were unexplored.
As scientists worked to unveil the ambiguity
surrounding electricity through inventions
and innovations, people grew more and
more intrigued. The Wimshurst Machine,
an example of these many magical inventions
caused an uproar as caused both the
scientific community and the general public
to wonder how it was able to produce
visible displays of electricity up to six
inches away on a standard model.
The theory of this machine lies within the
principle electrostatic induction, or static
electricity. In order for there to be any
presence of static electricity, there must
first be unbalance charges. In the last 100
years, it has been learned that these charges,
balanced or unbalanced, depend on the atom itself.
The Law of Conservation of Charge states that
all net charge is equal to zero, meaning
that while charges may freely be moved
around, they cannot be created or destroyed.
These machines belong to a class of electrostatic generators called
influenced machines, which separate electric
charges through electrostatic induction, or influence,
not depending on friction for their operation.
Earlier machines in this class were developed by
Wilhelm Holtz (1865 and 1867), August Toepler
(1865), J. Robert Voss (1880), and others.
The older machines are less efficient and exhibit an
unpredictable tendency to switch their polarity. The Wimshurst
does not have this defect.
In a Wimshurst machine, the two insulated discs and
their metal sectors rotate in opposite directions passing
the crossed metal neutralizer bars and their brushes.
An imbalance of charges is induced, amplified, and
terminals. The positive feedback increases the accumulating
charges exponentially until the dielectric breakdown voltage
of the air is reached and an electric spark jumps across the gap.
collected by two pairs of metal combs with points placed
near the surfaces of each disk.
These collectors are mounted on insulating supports and
connected to the output
The machine is theoretically not self-starting,
meaning that if none of the sectors on the discs has
any electrical charge there is nothing to induce charges on other sectors.
In practice, even a small residual charge on any sector
is enough to start the process going once the discs start to rotate.
The machine will only work satisfactorily in a dry atmosphere.
It does require mechanical power to turn the disks against
the electric field, and it is this energy that the machine converts into electric power.
The steady state output of the Wimshurst machine is a direct
(non-alternating) current that is proportional to the area covered
by the metal sector, the rotation speed, and a complicated function
of the initial charge distribution. The insulation and the size
of the machine determine the maximum output voltage that can be reached.
The accumulated spark energy can be increased by adding
a pair of Leyden jars, an early type of capacitor
suitable for high voltages, with the jars’ inner plates
independently connected to each of the output terminals and the
jars’ outer plates interconnected. A typical Wimshurst machine
can produce sparks
that are about a third of the
disc's diameter in length and several tens of microamperes.
In practice slight variations in the disc rotation rates
(e.g. due to belt slippage) smooth the output to a steady
increments to the Leyden jar charge. The available voltage
gain can be understood by noting that the charge density on
oppositely charged sectors, between the neutralizer bars, is
nearly uniform across the sectors, and thus at low voltage,
while the charge density on same charged sectors,
approaching the collector combs, peaks near the sector edges,
at a consequently high voltage relative to the opposite collector combs.
"Wimshurst Machine." Wimshurst Machine. Kenyon College. 05 Mar. 2014 <http://physics.kenyon.edu/EarlyApparatus/Static_Electricity/Wimshurst_Machine/Wimshurst_Machine.html>.
University, Princeton. "Wimshurst." Princeton University. 13 June 2013. Trustees of Princeton University © 2014. 05 Mar. 2014 <https://www.princeton.edu/ssp/joseph-henry-project/oscillatory-discharge/wimshu
