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Heated argument

Q: Why is it that when I get into a bath at 39degree C I feel totally
relaxed and yet, when I enter a room at the same temperature, I feel totally
stressed?

* * *

A: Although the bath may be at 39degree C, the air in the bathroom is
probably much colder, allowing part of the body to lose excess heat. This
heat loss is helped by evaporation of the bath water. In contrast, when
the air in a room is at 39degree C, heat will be flowing into one’s body,
rather than away, and body temperature will begin to increase. Since the
body then has to lose excess heat, the feeling of stress goads the brain
into taking action, such as drinking a cold drink or moving to a cooler
room.

Keith Lawrence Staines, Middlesex

Moonbeams

Q: Why does the Moon appear as bright as a cloud in the midday sky,
when it is a very dark body with an albedo of 0.07? The albedo of clouds
is around 0.6 to 0.8. (Albedo is the ratio of the intensity of light reflected
from an object to that of the light it receives from the Sun.)

* * *

A: The albedo of the darker areas on the Moon is indeed around 0.07,
but that of the mountains, rayed craters and cratered highlands is considerably
higher (between 0.10 and 0.15). As viewed in a bright blue sky through
binoculars or a small telescope, the dark areas appear almost indistinguishable
from the surrounding sky.

Mike Dworetsky Department of Physics and Astronomy University College
London

* * *

A: The light we see reflected from the Moon is just that, reflected
light. But we see the clouds by transmitted light because we are below
the clouds and sunlight is passing through the clouds, not reflecting from
them. Only high-flying jet travellers see light reflected from the tops
of clouds and, even through the tinted aircraft windows, the clouds are
10 times as bright as the Moon, as the albedo figures suggest.

Hazel Beneke Gatton, Queensland

* * *

A: Clouds have to be quite thick to reflect most of the sunlight. Unfortunately,
when they are sufficiently thick, they usually cover the whole sky and their
bright tops cannot be seen. The converse is also true: when the sky is only
partially covered with clouds, they are usually thin and no brighter than
the Moon. However, if it happens that there are well-developed but sparse
cumuli in the sky, and the Sun and Moon are on the opposite sides of the
sky, then it can be observed that the top of a large cumulus is about ten
times brighter than the Moon, as expected from their albedo ratio.

Leszek Fraskinski J. J. Thomson Physical Laboratory University of Reading

Round the twist

Q: I have often observed defects, or knots, in helical telephone cords.
The areas to either side of the defect show opposite helicity (see figure).

Strangely enough, there is often only one such defect in the cord, although
two defects, enclosing one anomalous region, would be expec- ted. It takes
a considerable effort to untangle the cord, which seems to be the only way
to restore uniform helicity. How do these defects hap-pen and how can they
occur spontaneously during normal handling of the telephone?

* * *

A: The telephone cord is ‘set’ into a helical form during manufacture.
Despite its structure being twisted, the cord is stable in that configuration.
When the user rotates the handset in the opposite way to the direction of
the helix, some of the in-built twist is removed. However, the de-twisted
cord does not become straight, but forms a length of helix in the opposite
sense. The greater the reverse rotation of the handset, the greater the
length of reverse helix. At the point where the two helixes meet, there
is a short region where the twist direction reverses. This is the apparently
hooked section which is irritating to telephone users. If one rotates the
handset in the direction of the original helix, until all the reverse-twist
has been removed, the reversal will soon disappear.

This phenomenon can be seen in yarns whose characteristics are modified
in the false-twist textile process, which produces continuous-filament yarn.
Twist is inserted into a bundle of filaments, the helical form being stabilised
by heating. The twist is then removed. The filaments do not become straight;
instead they form helixes of rapidly-reversing sense, much like the telephone
cord in question. The helixes give the yarn its characteristic stretch properties.
Unlike the single telephone cord, textile yarns are usually multi-filament.
The detail is therefore rather more complex, but the principle is the same.
You can see the same effect under a microscope using an old pair of stretch
tights.

Graham Waters Pontypool, Gwent

* * *

A: Knots in telephone cords are born as metastable knot-antiknot (k-a)
pairs when the cord is locally twisted against its natural coil. With a
little practice you can resolve a k-a pair into its components, and propagate
one element (say the antiknot) to the end of a new cord, where it can be
annihilated by twisting the handset. This adds a global twist to the lead
and leaves a stable knot in the middle of the cord. In normal use, dormant
global twists (gts) are induced by random movements of the handset. They
are barely detectable against the natural background, since their only effect
is to change the torsional energy of the cable and the total number of turns
from end to end. The chance encounter of a dormant gt with a k-a pair (formed
by an idle hand) then generates an apparently spontaneous and very stable
knot. On a larger scale, you might create an entire observable universe
by annihilating spontaneous antiparticles. God doesn’t play dice, He fiddles
with phone cords.

Alan Calverd Bishop’s Stortford, Hertfordshire

* * *

A: The effect occurs spontaneously in my household because the receiver
undergoes a half twist every time the phone is answered and a further half
twist when the receiver is replaced – thus gradually uncoiling the wire
until it can stand no more.

Oddly enough, with my telephone the helicity is the same on both sides
of the defect. The picture shown resembles the unstable state, from which
the coil could either return to the ‘normal’ position, or to a further state
which resembles a knot. However, as stated in the question, the helicity
changes across the defect.

Perhaps this is just another example of parity violation – some phones
change helicity, others (like mine) don’t. This could be a ground-breaking
area in physics research – if only we could find a macroscopic equivalent
of the top quark or tau neutrino!

Mark Burbidge Birmingham

This week’s questions

It’s a cracker: Why does the end of a whip crack?

David Innes Farnham, Surrey

* * *

Lemon twist: When I make a cup of tea, the liquid appears dark brown.
If I add lemon juice, the tea becomes a lighter brown with a yellow tinge.
What is the reaction taking place?

Anon