Hint: with just a piece of plastic and fur you can check your voltmeter. Now test the zinc-plate battery with your polarity-calibrated voltmeter. To verify your voltmeter's polarity, compare it to another commercial voltmeter! :) Or, connect it to a zinc/copper/saltwater battery (where the zinc is always the negative plate.) To verify that zinc battery-plates are the negative ones, use your voltmeter to discharge a capacitor where the capacitor dielectric is a sandwich: a layer of rubber and a layer of glass between adhesive metal plates: touch them firmly together and separate, and the metal connected to the rubber side will charge up negative, the glass positive. To determine the polarity of your commercial power supply, just look at the colored terminals, or check it with your voltmeter. (Rub glass against amber, and the "vitreous" glass becomes positive, while the "resinous" amber negative.)įor the Cathode Ray experiment, the positive plate would be connected to a positive terminal of a high-volt power supply. The marked +- on an ammeter was determined by permanent magnet poles and the coil winding direction.Īctually it was Ben Franklin who assigned the "positive" label to the rubbed fur's charge, since earlier it had been called Vitreous.
You could check polarity with the d'Arsonval Galvanometer you'd bought. Or fifty years later, the same standard was adopted when non-impulsive currents had been discovered and current meters were being sold. To determine electric polarity, just check the +- using your electrometer bought from an 1800s science supply company. These are voltmeters based on moving capacitor plates, and the ones with fixed polarity would use high-volt batteries to charge their pos. Early electrostatic voltmeters, the " Quadrant Electrometers," followed this standard. Simple: electrostatics is part of electromagnetism in general, and the names of the charge polarities come from electrostatics history.įur touched against rubber becomes net-positive while the rubber becomes net-negative.
Answering the first part: how was polarity first arrived at?
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