Ayrtha

Ayrtha is a respectful pixel portrait of Hertha Ayrton, after a Bain News Service photograph of about 1900, holding a white-violet carbon arc above a basin of sand ripples.

The Physics

Hertha Ayrton (1854–1923) showed that the hissing and instability of carbon arc lamps came from oxygen reaching the hot crater of the positive carbon, and summed up her arc work in The Electric Arc (1902). There she gave an empirical law for the voltage across an arc, now called the Ayrton equation. Her ripple paper on how vortices build sand ripple marks, “The origin and growth of ripple-mark”, was read before the Royal Society on 16 June 1904. In 1906 she became the first woman to receive the Society’s Hughes Medal.

V = a + b l + (c + d l) / I

• V: voltage across the arc (V)
• I: arc current (A)
• l: arc length, the gap between the carbons (m)
• a, b, c, d: empirical constants fitted for given electrodes (units V, V m⁻¹, W, W m⁻¹)

The (c + d l)/I term makes V fall as I rises, a negative differential resistance, which is why an arc needs a series resistor to stay stable. Sources: H. Ayrton, The Electric Arc (1902); Proc. R. Soc. Lond. 74, 565 (1904), doi:10.1098/rspl.1904.0159, and full paper Proc. R. Soc. A 84, 285 (1910), doi:10.1098/rspa.1910.0076; MacTutor, “Hertha Marks Ayrton”; English Heritage blue plaque.

Honest caveat: The Ayrton equation is empirical, fitted to carbon arcs; it is not derived from plasma physics and does not hold for every arc. How sand ripples grow and choose their spacing is still studied.

In character

They said the arc hissed because it was fickle. It hissed because it was breathing oxygen. I listened until it told me why.

On the hoodie

Front: a tiny arc between two rods over three ripples. Back: a pixel portrait of Hertha Ayrton with “CROWNED IN HER OWN LIGHT” and “HERTHA AYRTON - 1906”. Gildan 18500 Heavy Blend hoodie, Black, S to 5XL.

Part of the Unsung Hands set with Sponara, Recoilwyn. Browse all physics hoodies, more electromagnetism designs, or the Deep Cuts.