Robert N. Varney

Contact potentials between metals: History, concepts, and persistent misconceptions

Some early and not easily available history of contact potentials between metals is presented. A discussion is given of how the existence of a characteristic potential difference between metals was established. This discussion presents some idealized experiments and then analyzes the historic work in terms of them. From experiments, it is shown that the measured potential difference lies between two points that are respectively just outside of the two metals in contact. Certain points are brought out that are not often found in the literature. (i) Bennet discovered contact charging of dissimilar metals significantly earlier than Volta did. (ii) Nothing in 19th (or 20th) century physics discloses experimentally the internal potential difference between two dissimilar metals in contact. (iii) Many 19th century physicists, including Maxwell, thought that the Peltier effect disclosed the value of the internal potential difference between dissimilar metals in contact. Later analysis showed that Maxwell was wro...

Electromotive force: Volta’s forgotten concept

The concept of electromotive force as first presented by Volta seems to be all but forgotten. Introduction of the term often occurs without any definition at all and is often confused with electrostatic potential difference. When a formal definition is given it is usually neither in accord with Volta’s original idea nor conceptually useful or comprehensible. We emphasize Volta’s use of the term to describe nonelectrostatic action on charges. The relationship of emf’s to electrostatic potential differences is presented through the introduction of electrostatic and nonelectrostatic fields (as was described by Abraham in 1904). A failure to note a distinction between these types of electric fields mars some highly important works including J. C. Maxwell’s.

Electric fields associated with stationary currents

The solution to a puzzle proposed by Merzbacher in this Journal [Am. J. Phys. 48, 178 (1980)], the determination of the electric field outside a long straight wire carrying a constant current, is discussed.

Falsification by space charge of electron beam energy measurements

When the energy of the electrons in a beam is determined by noting the value of a stopping potential that cuts off the beam, the current density of the beam must be below a limiting value determined by space charge considerations. Above this limit, cutoff occurs sharply at a voltage that may be far below the potential that accelerates the beam. At stopping potentials above cutoff, a weak tail of current does persist all the way up to the full potential of the beam.

Mean Free Paths, Ion Drift Velocities, and the Poisson Distribution

In elementary kinetic theory of gases, the topic of free paths is usually limited to the simplest case of finding the probability of zero collisions in a travel distance x. This leads to a mean free path and it also discloses that the greatest probability of collision in a distance Δx occurs for x = 0. The most probable free path then does not coincide with the mean free path. This paper carries on the analysis to evaluation of the probability of exactly one collision in a distance x and still further to the probability of exactly n collisions in x. All of these probabilities, for which the number of collisions n is greater than zero, are bell-shaped curves with the most probable value of x coinciding with n times the mean free path. The results are applied to analyze the sharpness of the observed drift velocity of ions moving through a gas in an electric field.

Physics in Engineering—A Rejoinder

The contentions are raised that a thoroughly integrated presentation of fundamentals is highly essential to engineering students and must be given regardless of the title or department of the professor doing it, that physics departments and texts are doing it today, not perfectly perhaps, but under considerable difficulties, and finally that physicists still have an approach and a point of view that is decidedly more basic than that of the engineer and is decidedly healthy for the engineering student to learn. A suggestion is offered for expanding the crowded four-year curriculum by bringing students from high schools a year earlier.