Pearson–Anson effect

The Pearson–Anson effect, discovered by Stephen Oswald Pearson[1][2] and Horatio Saint George Anson[3], is the phenomenon of an intermittent electrical current through an electrical load exhibiting S-shaped negative resistance (e.g., a neon lamp) connected in parallel to an accumulating element. In Pearson and Anson's original circuit, a neon lamp is connected in parallel to the capacitor of an RC circuit that is permanently supplied by a voltage source.

Contents

Operation

Charging

The power supply continuously charges the capacitor through the resistor. As the neon lamp exhibits negative differential resistance in the A–B part of its I–V curve (see the figure on the right), if it is driven by a voltage source (the charged capacitor here), it behaves as a switching element with hysteresis (a sort of Schmitt trigger).

Once the voltage across the lamp exceeds a critical value (the high voltage threshold) at point A of its I–V curve, the current through the lamp "jumps" up to a higher value (the point C that stays exactly above the point A); the voltage across the lamp does not change in this instant.

Discharging

The capacitor quickly discharges through the neon lamp (that also absorbs the whole current flowing through the resistor) and the voltage across the capacitor quickly decreases (a small resistor may be added in series to the lamp to limit the current and to extend the discharging duration). When it reaches the lamp's critical voltage (the low voltage threshold) at point B, the current through the lamp "jumps" down to a lower value at the point D directly beneath the point B – the voltage across the lamp remains constant in this instant.

As the current flowing through the neon lamp is too small to keep it conductive, the lamp extinguishes and the current from the power supply begins recharging the capacitor until the cycle repeats. The circuit therefore exhibits a low-frequency relaxation oscillation, during which the lamp flashes each time it conducts.

It is interesting fact that, in this old-fashioned arrangement, the humble neon lamp with RC circuit performs all the functions of the contemporary electronic 555 timer plus flashing! That is why this circuit is referred to as "elegant simplicity"[4]

References

Footnotes

  1. ^ Stephen Oswald Pearson,Dictionary of Wireless Technical Terms (London: Iliffe & Sons, 1926).
  2. ^ Frederick Walter Baynes and Stephen Oswald Pearson, "Improvements in advertising and display apparatus," British patent GB201374 (filed: 5 July 1922). Available on-line at: http://v3.espacenet.com/publicationDetails/originalDocument?CC=GB&NR=201374A&KC=A&FT=D&date=19230802&DB=EPODOC&locale=en_EP .
  3. ^ Horatio Saint George Anson (1903-1924(?)). Son of Rear-Admiral Charles Eustace Anson, Superintendent of the Royal Navy’s Chatham Dockyard, and Maria Evelyn Ross. [Genealogical information: Marquis of Ruvigny and Raineval, The Plantagenet Roll of the Blood Royal… The Anne of Exeter volume (London: T.C. & E.C. Jack, 1907), page 92.] As a youth in the Royal Naval College, he developed an interest in research into radio. He subsequently joined Faraday House, the headquarters of the Institution of Electrical Engineers (now the Institution of Engineering and Technology), in London. There he discovered that the negative resistance of neon lamps could be exploited to create an oscillator. In 1924 he was appointed to the Research Department of the Royal Aircraft Establishment in Farnborough, England. He was immediately elected a fellow of the Physical Society of London. However, soon thereafter he died in a car accident. [See: Commander Frank W. Lipscomb, The Wise Men of the Wires: The History of Faraday House (London: Hutchinson, 1973), pages 96-97. ]
  4. ^ Elegant simplicity by D. Lancaster

See also