Posted by: Rob Viens | July 22, 2012

Watching a 19th Century “Plasma Screen”

On July 22nd, Darwin was treated to an electrical show as he neared the entrance to the Rio de la Plata.  He writes:

“We have had this morning a true specimen of the Plata weather.— The lightning was most vivid, accompanied by heavy rain & gusts of wind.— The day has been exceedingly cold & raw.— We passed through large flocks of different sea-birds.— & some insects & a bird very like a yellow hammer flew on board.— We are about 50 miles from Cape St Marys.— I have just been on deck.— the night presents a most extraordinary spectacle.— the darkness of the sky is interrupted by the most vivid lightning.— The tops of our masts & higher yards ends shone with the Electric fluid playing about them.— the form of the vane might almost be traced as if it had been rubbed with phosphorus.— (Note in margin: St Elmo’s fire.) To complete these natural fireworks.— the sea was so highly luminous that the Penguins might be tracked by the stream of light in their wake.— As the night looked dirty & there were heavy squalls of rain & wind, we have dropped our anchor.” (July 22)

As he noted in the margin of the diary, along with the lightning strikes, Darwin was getting to experience something most seasoned sailors where familiar with – St. Elmo’s Fire.

“St. Elmo’s fire on masts of a ship at sea” – 1866 (from Wikipedia Commons):

St Elmo's Fire

St. Elmo’s Fire occurs when you put a long (ideally pointed) object (like a ship’s mast, lightning rod, or airplane wing) in an electromagnetic field (i.e., a thunderstorm). The thunderstorm creates an electromagnetic field that gets concentrated around the mast.  Where the mast comes to a point (say at the top), the field is the most concentrated. The air molecules (nitrogen and oxygen) in the concentrated field become ionized (i.e., have their electrons striped away). This creates a “cloud” of ionized atoms, called plasma, which conducts electricity and glows. The brightness of the glow depends on the strength of the concentrated electromagnetic field. Hence pointed objects (with a more concentrated field) tend to intensify the glow.

Glowing plasmain a a plasma globe (from Wikipedia Commons)
Plasma Globe

You may be more familiar with some of the practical uses of “glowing” plasma – say plasma screen TVs, or fluorescent lights and neon signs.

The phenomenon is named after St. Elmo (St. Erasmus of Formiae), the patron saint of sailors.

St. Elmo’s Fire on an airplane wing (from meteoboy.hubpages.com):

St Elmo's Fire

In his Narrative, FitzRoy also describes St. Elmo’s Fire – a phenomenon that he probably had a lot of experience with:

“Just before the rain began, St. Elmo’s fire was seen at each yard-arm, and at the mast-head. Those who have not seen this light, always a favourite with sailors, because they say it only appears when the worst part of the storm is over, may excuse my saying that it resembles the light of a piece of phosphorus—not being so bright, or so small, as that of a glow-worm, nor yet so large as the flame of a small candle. I was curious enough to go out to a yard-arm and put my hand on a luminous spot; but, of course, could feel nothing, and when I moved my hand the spot reappeared.” (Narratives, FitzRoy)

Lightning over the sea (from colourlovers.com):
lightning at sea

He also has a great description of lightning obliterating the mast of the HMS Thetis, while he was serving on the ship in the vicinity of Rio. As FitzRoy’s descriptions are always interesting, I thought I’d include that description here, too.

“Many ships and buildings have been struck, during late years, still there are but a very few protected by lightning conductors. I was a lieutenant on board the Thetis, when her foremast was shattered by lightning, in Rio Harbour, and shall not easily forget the sensation. Some of the officers were sitting in the gun-room, one very dark evening, while the heavens were absolutely black, and the air hot and close, to an oppressive degree, but not a drop of rain falling, when a rattling crash shook the ship. Some thought several guns had been fired together—others, that an explosion of powder had taken place; but one said—”The ship is struck by lightning!” and that was the case. The top-gallant masts were not aloft; but the fore-topmast was shivered into a mere collection of splinters; the hoops on the foremast were burst, and the interior, as well as outside of the mast, irreparably injured. From the foremast the electric fluid seemed to have escaped by some conductor, without doing further damage; yet it filled the fore part of the ship with a sulphureous smell, and the men who were there thought something full of gunpowder was blown up.

No person received injury: the foremast was taken out afterwards, and replaced by another, purchased from the Brazilian government at a great expense, and made by the carpenters of the Thetis. I should say that the electric fluid shook rather than shattered the fore-topmast, for it did not fall, but resembled a bundle of long splinters, almost like reeds. It twisted round the head of the foremast, instead of descending by the shortest line, went into the centre of the spar, and then out again to the hoops, every one of which, above the deck, was burst asunder. The Thetis was to have sailed in a few days, but was detained by this accident almost two months. She had no conductor in use.” (Narratives, FitzRoy)

It is no wonder that FitzRoy spared no expense to get the most up-to-date lightening conductors installed on the Beagle before he left Plymouth.  He knew the risks, and the potential price, of not taking the threat of natural electricity seriously. (RJV)

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Responses

  1. […] to the South & North of it are comparatively rare.” (April 24) (See A Mighty Pampero and Watching a 19th Century “Plasma Screen” for more on these meteorological […]

  2. […] to the river – welcoming him to Uruguay when he first arrived there almost 1 year ago (see Watching a 19th Century “Plasma Screen”).  One of the striking things about lightning (no pun intended J), is that it is so ephemeral – […]


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