Past exam of the medicine course of the University of Oxford / 1864 / First examination / Physics / 10

What evidence is there of the identity of frictional and of Voltaic electricity ?

Frictional electricity and voltaic electricity name different methods of producing electrical effects. Their identity is supported by showing common kinds of response and quantitative continuity between them, rather than merely observing that both can give a sensation.

Charges from either source show the same attraction and repulsion, can charge a conductor or capacitor, and can discharge through a circuit. A current from either source can generate a magnetic field, produce resistive heating, and cause appropriate electrolysis. Reversing the sign or current direction reverses the corresponding directional effects. Different observed strengths need not imply different electrical substances.

Michael Faraday carried out a comparison of common and voltaic electricity. In a specific magnetic test, he discharged a machine-charged jar battery through a galvanometer, using a wet thread in the discharge path; the needle's deflection connected ordinary static charge with a current's magnetic action. He also compared common and voltaic electricity quantitatively.^[ref]

The modern distinction is mainly in the source characteristics. A friction machine commonly supplies a small charge at high electric potential difference, while a chemical cell can sustain a much larger continuing electric current at a lower voltage. An insulating gap or a poorly conducting path can therefore affect the demonstrations differently. Once charge passes through a specified circuit, its electric and magnetic action is not determined by whether mechanical rubbing or a chemical reaction originally supplied the energy.

For a specified electrode reaction with controlled current efficiency, Faraday's laws of electrolysis express the quantitative relationHere $M$ is the product's molar mass, $z$ the electrons per reaction unit and $F$ the Faraday constant. Equal transferred charges give equal reaction equivalents under those conditions. Charge, voltage and current must be distinguished: equal voltages alone do not imply equal delivered quantities of electricity.