Julius Sumner Miller: Lesson 39 - Adventures with Electric Charges

A - The ELECTROPHORUS: We have in this device a remarkable scheme of things. A lucite slab is rubbed or slapped with a cat's fur. This WORK separates charges. Now we put on the lucite slab a metal plate equipped with an insulating handle. We touch the upper surface of the metal plate. This grounds this face of the plate! Row we lift the metal plate away from the lucite slab. This is now quite hard to do since the Coulomb forces are very large. But doing it requires WORK again and this energy shows itself in the availability of an electric spark. With this energy we can light a fluorescent lamp! And - a wonder to say: We can continue to take energy from this system FOREVER - which is a very long time! It is not that we have a Perpetual Motion Machine - no - never that - but rather that work is required to separate the plate from the slab -and work and energy are synonymous.
B - The Smoke Precipitator: A glass tube is fitted up with electrodes. We put some smoke in the tube. We connect the electrodes to a van de Graaff generator - a device for producing a large electric spark. Instantly the smoke disappears. The reason: the smoke is made up of charged stuff in abundance. When the van de Graaff is turned on large electric field arises between the electrodes whereupon the charged stuff migrates to the charged terminals - moving under the action of the electric field.
C - A three-vaned device has pointed ends - sharply pointed. We place it atop the sphere of a van de Graaff. Charges move to the sharp points. This accumulation of charge - which becomes very dense at sharp points - gives rise to a charge migration from the sharp points to ions in the air. These ions are abundant at all times - due to cosmic radiation. In addition - some are brought about by the intense electric field in the region of the points. The reaction forces turn the spin-wheel.
D - The Mad Professor's Head: An array of slips of paper are fitted to a stand. The system is put atop a van de Graaff. The charges move to the paper strips and because all the strips now carry the SAME charge the mutual repulsive forces drive the paper slips from each other. This can be done with the hair on your head - but be careful!
E - A candle flame is placed between two electrodes - one sharp pointed-the other a sphere. With charge from a van de Graaff it can be shown that the greatest charge density arises at sharp points. It is further seen that a flame contains ions in abundance. This brings to mind the classical report of Ben Franklin to The Royal Society wherein he recommended sharp points as lightning arrestors to protect homes and barns from "that mischievous thing called lightning".
F - We show a Dissectible Leyden Jar. This is a device made up of three parts - two conductors and an insulator. We can "store" electric charge in this thing with extraordinary results. With the inner conductor in contact with the charged sphere of a van de Graaff and the outer one grounded by hand we CHARGE the Leyden Jar - which we can call a condenser or a capacitor. Now if we connect the innermost conductor with the outermost a fat sharp spark is gotten - representing enormous energy. It is usual to say that the energy resides in the insulator which we call the dielectric.