The electrophorus of S. Kamachi

Thomas B. Jones
Professor of Electrical Engineering
University of Rochester


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.

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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.

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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.

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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.

NOTE: 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.

Library reference

S. Kamachi, "Introduction of educational implements for the prevention of hazards caused by ESD," Proceedings of ESA 1995 Annual Meeting, (Laplacian Press, Morgan Hill, CA) 1995, pp. 15-21.

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Last modified: Monday, 15-Mar-2010 08:57:30 EDT