ÐÓ°ÉÔ­´´

Dirty films

Q: When I clean the inside of my car windows, I notice that they are
covered with an oily film that leaves a noticeable black residue on the
cleaning cloth. Have others noticed this build-up on their car windows?
Can anyone suggest what might be causing it?

* * *

A: There are a lot of plastics used in the manufacture of a car, especially
around the inside of the windscreen. Most contain waxy plasticisers which
provide flexibility. These boil off in the heat under the windows and condense
as a sticky film which attracts fine particulate matter from the air intakes.
Another effect is that the plastics in a car become brittle with age as
plasticisers are lost.

W. T. Clements Hadleigh, Essex

* * *

A: The emission of plasticisers causes both the window film and the
smell of a new car. Old cars that have spent much time in the sunshine do
not have this problem because all the plasticisers have been lost. However,
they do have brittle plastic dashboards that tend to crack and splinter.
Investing in a car that has dashboards covered in leather should remove
both problems. This could be one of the less well-known reasons for buying
a Rolls-Royce.

Peter Brooks Bristol

* * *

A: I cannot suggest the precise source of this film, but thank you for
drawing my attention its existence. After cleaning it off with a damp cloth,
I was surprised at how much better I could see out.

Jack Willans Lancing, West Sussex

Light sneeze

Q: I have noticed that many people tend to sneeze when they go from
dark conditions into very bright light. What is the reason for this? (Continued)

* * *

A: The tendency to sneeze on exposure to bright light is termed the
‘photic sneeze’. It is a genetic character transmitted from one generation
to the next and which affects between 18 and 35 per cent of the population.
The sneeze occurs because the protective reflexes of the eyes (in this case
on encountering bright light) and nose are closely linked. Likewise, when
we sneeze our eyes close and also water. The photic sneeze is well known
as a hazard to pilots of combat planes, especially when they turn towards
the Sun or are exposed to flares from anti-aircraft fire at night.

R. Eccles Common Cold and Nasal Research Centre, Cardiff

Light fantastic

Q: Why does the filament in a standard household light bulb start glowing
when it is put into a microwave oven (and the oven is turned on)?

* * *

A: If a small, high-resistance antenna is placed in an environment awash
with electromagnetic radiation, its electrons vibrate at a higher frequency
than normal. This increased motion is dissipated through the filament, which
heats and glows. An even more impressive result can be achieved by placing
forks in the microwave.

Gary Hughes Australia

New ÐÓ°ÉÔ­´´ does not recommend the verification of either the light
bulb or the fork experiments as correspondence from readers has confirmed
that overheating the bulb can cause it to explode – Ed.

Mystery digits

Q: Computerised address labels on much of my mail contain the same
five-figure number, which means nothing to me, and is not related in any
obvious way to any element of my address. Placement of the number on the
label varies. I don’t think it can result from one organisation selling
a mailing list, since the labels originate from varied sources: subscriptions,
direct sales, charities, even the Inland Revenue. Can anyone explain what
this code might be?

* * *

A: This number is the Mailsort code. Mailsort is the latest stage of
an elaborate and bizarre practical joke played by the Post Office at our
expense. The first stage was postcoding, a vast and enormously costly 15-year
exercise (1959 to 1974) in which every British address was allocated an
alpha-numeric postcode. The rest of the world (except Canada) was content
to assign simple numeric codes, usually five digits, where the first one
or two digits represented the province or region, and the rest identified
the city and locality.

But the British Post Office would have no truck with this foreign logic
– it devised its own convoluted coding system. Post towns were assigned
‘mnemonic’ prefixes (such as FY for Blackpool and TG for Galashiels), and
then an incoherent mixture of letters and numbers was tagged on, giving
codes of wildly varying length and structure (GIR 0AA, S1 4BT, M16 0HQ,
HX1 2YL, LS23 6BW, EC4V 3PT and so on).

Since these codes were based on random alphabetical prefixes, they were
virtually useless for routeing mail. Letters for Halifax, Huddersfield and
Bradford are all sorted in Bradford, but their prefixes – HX, HD and BD
– give no indication of their geographical proximity. And, as optical character
reading equipment became available, the codes proved troublesome – S was
read as 5, Y as 4, M as N or W, and so on.

To be blunt, British postcodes simply didn’t work. But, having spent
millions on coding the entire country and persuading everyone to use them,
the Post Office couldn’t then drop the system overnight. So instead, it
quietly introduced a parallel coding system called Mailsort – and this is
the system that is actually used for the sorting and routeing bulk mail.

Mailsort is a five-digit code, like the foreign systems: one digit for
the region, two for the postcode centre and two for the local delivery office.
Companies which send large mailings (at least 4000 letters at a time) can
qualify for discounts of up to 25 per cent if they present their mail in
Mailsort order. If they are foolish enough to present their mail in postcode
order, they get no discount.

Thus we now have a two-tier postal service. Bulk mailers can benefit
by using the five-digit Mailsort system, while ordinary punters use the
officially-promoted alphanumeric system, and pay a much higher price.

Dermod Quirke Halifax, West Yorkshire

This week’s questions

Under the rainbow: I have twice driven through the end of a rainbow
where it meets the ground, although I am told this is impossible. I was
surrounded by the rainbow’s colours and there were many other drivers to
witness that it can happen. Can anyone explain this?

Phil Herbert Loughborough, Leicestershire

* * *

Happy not sad: Some years ago, between rain showers, I noticed an ‘upside-down’
rainbow (u-shaped rather than n-shaped). The colours were also reversed.
It appeared around 40 degrees above the horizon and was smaller than an
upright rainbow. It persisted in a semicircle for about a minute before
slowly fading from one side, the remaining arm lasting for another minute.
Can anyone explain this?

R. Dufton Banbury, Oxfordshire

* * *

Spray it again: I have a roof garden lit by a floodlight at night, and
have attempted to enhance the view with an artificial rainbow against the
night sky. But whatever combination of light and liquid sprays I have tried,
no colours appear. Why?

Nicholas Saunders London