Description
Shonosuke Kamachi, while working at the
Technical Institution of Industrial Safety in Tokyo, Japan, developed a new
apparatus to demonstrate electrostatic principles [Kamachi, 1995]. Over the years, he has used it
successfully in presentations to a variety of audiences, ranging from the
technical professionals for whom it was originally developed to high school
students and even young children. The apparatus is essentially an
electrophorus, cleverly adapted by mounting a combination of aluminum and
TeflonTM disks on
a hollow, insulating shaft. The disks, which are about 30 cm in diameter and interchangeable,
can be quickly attached at the end of the shaft, or mounted using a
sleeve so that they slide smoothly and freely on the shaft.
This simple design results in a remarkably adaptable
demonstration tool which can be used to illustrate many
electrostatic concepts and phenomena, including induction charging, capacitance,
the Franklin bell, the Franklin motor, vapor ignition, etc. To operate it, the apparatus is
assembled with the disks in the desired configuration and then grasped by the handle.
Tilting the shaft up or down causes the moveable disks to slide away from
or toward the fixed disk mounted at the end. Typically, a TeflonTM (or aluminum) disk is mounted at the end of the shaft and
a second disk is mounted so that it will slide back and forth, that is, toward or
away from the disk at the end. See the sketch above. Before mounting it on the shaft, the
TeflonTM disk is frictionally charged by rubbing
it vigorously with a cloth (acrylic or wool both charge the Teflon
negatively). With practice, the sliding motion of the disk can be accomplished
quickly and repeatably while holding the apparatus in one hand. This
apparatus can be operated in very much the same way as the classical electrophorus to charge up the aluminum
plate, which can then serve as a source of charge. But the apparatus can be set up in many other
useful configurations as well. For example, mount two aluminum plates on the shaft
at a spacing of 3 to
4 inches (10 to 12 cm). Now bring a charged Teflon disk (mounted on another shaft)
parallel and close to one of the electrodes and charge that electrode by induction. By this
procedure, one creates a parallel-plate capacitor with a fairly
strong, almost uniform electric field (~105 V/m).
Kamachi employs this configuration to demonstrate the
Franklin bell and Franklin motor. A simple yet dramatic demonstration of how
stored electrostatic charge can lead to an ignition hazard is depicted in the
photograph below. The setup uses one of the aluminum disks as a base to hold the
insulating shaft vertically and free-standing on a bench or table. Note that the aluminum disk mounted at
the top of the shaft is isolated from ground. After depositing a few drops of acetone,
alcohol, or some other flammable liquid on the upper disk with an eyedropper, the
freely sliding charged Teflon disk is used to charge the isolated electrode by induction.
Then, a small metal ball attached to a grounded metal rod is brought close to
the edge of the liquid puddle to draw a capacitive spark. If the discharge is
sufficiently energetic and if the vapor/air mixture is within the flammability
limits, the liquid will ignite as shown in the photograph. Unlike in a closed chamber, ignition here is a bit harder
to achieve. This is because the mixing conditions of vapor and
air above the puddle are almost impossible to control. The demonstrator will probably need to try repeatedly to
ignite the acetone; however, any such difficulty can be
turned to instructive advantage, providing an important lesson
about the fuel/air mixture requirements. To be specific, the ratio of vapor to
oxygen molecules in the region of the spark must be within a rather narrow range, close to
the stoichiometric limit, in order to make an ignition
possible. To achieve the goal of an
effective yet safe demonstration with this apparatus, only a small volume of
flammable liquid should be used; an eyedropper full is quite enough. Safety
glasses must always be worn by the demonstrator and by anyone else involved in
the demonstration. Also, be sure to move all containers of flammable liquid well
away from the apparatus before trying to ignite the liquid on the
electrode.
