Sundial shenanigans
Question: I left my watch exposed to the sunlight for a long time and
afterwards its LCD (liquid-crystal display) was completely black. However, when
I removed it from the sunlight and it cooled down, after a few minutes it was
back to normal again.
Could anyone tell me what happened to the display and how it recovered?
Answer: Liquid crystals used in most LCDs are long, straight molecules that
like to line up with each other, and anything else that is around. If you
sandwich a film of liquid crystals between glass plates that are are ridged like
a miniature corrugated roof, the molecules will line up with the ridges. If you
rotate one plate by 90掳 the molecules near that plate will orient themselves
at right angles to the molecules near the other plate. Between these plates, the
rest of the crystal lattice forms a smooth one-quarter twist. This twist rotates
the polarisation of light by 90 degrees as it passes through the liquid
crystal.
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The liquid crystals used in displays are electrically unbalanced: one end of
the molecule is slightly negatively charged, the other end slightly positively
charged. So applying a small voltage across the glass plates causes all the
molecules to 鈥渟tand on end鈥�, and the liquid crystal loses its ability to twist
the polarisation of light. Switch off the voltage, and the lattice returns to
its previous state.
To create a display, the glass plates are replaced with polarising filters,
also out of alignment by 90掳, and a reflecting surface is put behind them.
Incoming light is polarised by the first filter, twisted 90掳 by the liquid
crystals, passes through the second filter, is reflected and reverses its
journey.
Apply a voltage, however, and the incoming light passes unchanged through the
liquid crystal and so can鈥檛 pass through the second polarising filter.
Consequently, the display goes black. By using segmented electrodes, letters,
numerals and other shapes can be displayed.
The liquid crystal state is a phase between solid and liquid鈥攃ool it
and it solidifies; heat it and it melts. Melted liquid crystals lose their
ability to change the polarisation of light, becoming ordinary liquids. So when
liquid crystal displays are heated they become, in effect, two sheets of
Polaroid out of alignment with a slightly sticky transparent liquid between
them, so the entire display goes black. When cooled, it returns to the liquid
crystal phase and reflects light again.
Jon Bowen
Elsfield, Oxfordshire
Fast burn
Question: In March 1727, Mary Delaney attended the coronation of King George
II in Westminster Hall. She later wrote: 鈥淭here were 1800 candles besides those
on the table, and all were lighted in three minutes by an invention of Mr
Heidegger鈥檚.鈥� Who was he and what was his invention?
Answer: I have found a reference to the lighting of the candles at the
coronation feast of George III on 22 September 1761, where as many as 3000
candles were lit in less than half a minute.
According to the poet Thomas Gray, the Queen and her ladies 鈥渨ere in no small
terror鈥� as trains of flax were set alight and flames ran swiftly from candle to
candle. The expiring flax apparently fell in large flakes upon the heads of
those beneath but, fortunately, did no harm.
This may have been a rerun of the candle-lighting arrangements at the
coronation of the king鈥檚 grandfather, George II, but there is no reference in
Westminster Hall by Hilary St George Saunders, the source of the quote
used by your questioner. However, as Heidegger died in 1749 it seems likely.
John James Heidegger was a Swiss citizen who moved to London to pursue a
career as an opera impresario. He was manager of the Haymarket Opera House in
1713 and later worked with Handel. Having patronised Heidegger鈥檚 masquerades,
George II appointed him Master of the Revels, although a Middlesex grand jury in
1729 was not so charitable, presenting him as 鈥渢he principal promoter of vice
and immorality鈥�. In his defence, however, there is evidence that he gave
generously to charity.
Apparently George II invited himself to Heidegger鈥檚 home and complained at
finding it in darkness. Heidegger affected an apology and, as he was speaking,
the house was lit up instantly by 鈥渁n ingenious arrangement of lamps鈥�. This,
presumably, was the invention referred to by your correspondent, but there
remains the question as to why he should try to impress George II if the king
had already seen it in operation at his coronation.
Geoff Embleton
Deputy Establishment Officer
House of Lords, London
This week鈥檚 questions
Dirty business: In James Bond films, a gun with a silencer is used to dispose
of bad (and good) guys. How does the silencer work?
Jeremy Charles
Chesham, Buckinghamshire
New star: How large will the International Space Station have to be before it
can be seen with the naked eye? And when built, how significant will it appear
to an observer on Earth?
Alf Cousins
One Tree Hill, South Australia