I get quite a few letters and emails, many of them asking some
very good questions. I thought I would share some of these
with you:
Q. I was intrigued when, in a
recent broadcast of What the Ancients Did for Us (great programme!),
you mentioned the Baghdad Jar, apparently an ancient way of
generating electricity. You seemed to have some
difficulty in deciding what they might have used the electricity
for.
For some time I have had this crazy idea that the ancients might
have used hydrogen balloons to help them lift the enormous stones
that
they used to build many monumental structures. Crazy, of course.
Where
on earth would they have got their hydrogen from? And anyway, some
television presenter (it might have been you: I forget) has already
demonstrated how the use of kites may have been beneficial in
lifting
stones. So, no need for balloons?
But, with electricity, and therefore electrolysis, hydrogen might
have
been produced. Is my balloon idea still so crazy? Probably! But you
might consider, in a future programme, trying to make a balloon out
of
animal skins, filling it with hydrogen, and seeing what it could
lift.
Could be fun?
- J. Trevor Catlow
A. An ingenious idea, but highly improbable, for three
reasons. First the Baghdad battery generates half a volt at best,
and is highly susceptible to build-up of Back-EMF [reverse voltage]. To electrolyse
water you need 1.23 volts. I doubt if you could ever get a sustained
voltage as high as 1.23, since when you put a lot of batteries in
series the Back-EMF is horrific.
Second, hydrogen is highly mobile, and leaks rapidly through most
soft materials. I don't think it would be possible, even today with
modern materials, to make a balloon from animal skins that would
hold hydrogen.
Third, if you wanted to lift say a 1-ton stone you would need a
hydrogen balloon perhaps 20 metres in diameter. That is an awful lot
of animal skins to stick together. And a 10-ton stone would need a
balloon 40 metres high. It's not on.
Nice try, though.
A. Yes the period of pendulum is governed by its length
and is independent of the mass at the end. But if you add an old
penny to the bob of the pendulum of Big Ben, then you slightly alter
its length; in fact you make it a bit shorter, because the centre of
mass of the bob is slightly raised. Similarly if you take pennies
off, you will in effect slightly lengthen the pendulum. This seems
an odd way to do it, but it is obviously very convenient, and
simpler than winding the bob up and down by a minute amount.
Q. Why can I tear a paper tissue
one way only in a straight
tear downwards, if I turn it round and tear then its a very ragged
result?
- Mrs J C Phipps, Kidderminster
A. The reason why a paper tissue will tear tidily in one
direction and not the other is relatively simple. Paper is made by
laying down fibres in one direction, like the warp on a piece of
woven cloth. Fibres are then dumped in roughly the other direction
from a suspension in water so it is like shaking a lot of parallel
strings in a dish of spaghetti. Some of the spaghetti will lie
across the strings and some all over the place.
When it is all dried out the paper looks even, but in practice it
is highly directional, it will tear easily along the line of the
original fibres, because there are weaknesses between them. However
there is no particular direction across, so it will normally tear
jaggedly. This is also very obvious with newspaper. If you take a
broadsheet newspaper, it will normally tear very easily from top to
bottom, but trying to do it from side to side is much more
difficult.
Q. How does fresh air come into an open window, at the same
time that stale air goes out?
- Mrs J C Phipps, Kidderminster
A. When you have a window open in a room you might think
that no air would go in or out. However there are two things
going on that make fresh air come in. First there are random changes
in air pressure. If you were to measure the air pressure just
outside the window you would find it went up and down every minute
or two. These changes in air pressure cause winds to blow, and they
depend upon the heating of the earth's surface by the sun. So when
there is a slight reduction in pressure outside, air from the room
will flow out through the window. When the pressure outside gets a
little higher, air will flow in again. Secondly, even when there are
no changes in pressure the air is always moving slightly. Only very
rarely is it absolutely still. If the air is moving both inside and
outside the room inevitably some will flow out and some will flow in
and it may go like waves, first out then in or it may flow out
at the top of the window and in at the bottom. Therefore even with
quite a small window the air in the room will gradually get replaced
with fresh air.
Q. On Science Shack you
heated a bath with cold water from a river ... how on earth did you
do that?
- Luke Hutson, West Norwood
A. This is roughly how we heated the bath.
We took a fridge, and filled it with cold water
from the river. Then we switch it on, so that the water
inside got a bit colder, and the coil on the back of the fridge
got warm. We used this warm coil to add a little heat to the
water for my bath. Then we returned the water to the river,
a little bit colder than it was when we collected it. Then
we repeated the process, to put a bit more water back in to
the river a little colder than when it came out, and to warm the
bath water a bit more.
To make things simpler, we took the water from the
river in a hosepipe, a pumped it continuously through the fridge,
so that there was a stream of water coming out of the river, and
going back in a little bit colder (about 2c colder, I
think). Meanwhile the bath water got warmer and
warmer. We stopped the process when the bath water reached
40c, because I did not want to be poached...
I hope this makes sense. I did not believe
it until I saw it with my own eyes. In principle anyone
could use this method to heat their house, taking heat either from
a stream or from deep underground.
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Q.
I understand how before the transit of Venus all the astronomers
had were a lot of angles from which they could work out the relative
distances but no actual distances. They only needed one distance to
work out all the others. I was able to follow how in Captain
Cook's time, by observing the transit from different, widely spaced
positions on the Earth, they were able to work out the distance of
the Earth from the Sun.
However, I cannot work
how Jeremiah Horrocks make his calculation of 59 million miles for
the astronomical unit (the Earth-Sun distance)? As Jeremiah
was observing from only one position I cannot see how he had
sufficient information to work anything out. It has been
suggested to me that the observation of the times of contact: the
T1, T2, T3 and T4 times should help but I can't see how you can get
from these times to give you a distance- I can only see a load of
angles and no distances.
- Keith Wells, Brighton
A. I went to my friend Prof David Hughes, The University of
Sheffield who provided us with this most complete answer!
"You are absolutely correct about your conclusion, i.e. that
Jeremiah 'calculated' a value of 59 million miles (14733 Earth
Radii, 94 million km) for the astronomical unit (the Earth-Sun
distance).
(i) Horrocks knew that the distances between Mercury, Venus,
Earth, Mars etc and the Sun where in the ratio
0.387:0.723:1.000:1.523 etc
(ii) He also knew that, at inferior conjunction, subtended an
angle of 76 seconds of arc at the Earth. This was (to him) the
important result of his Much-Hoole transit observation. The
angular diameter of Venus had not been measured before, due
to the problem of irradiance (i.e. the image of Venus was much too
bright against the background) and the fact that the micrometer had
not been invented.
(iii) he also knew that planet Earth had a radius somewhere
between 5800km and 6900km (I am not sure of whose value he used,
there were a few about at the time, the two I give are about 10%
each side of the correct value.)
(iv) Now for the naught bit. Horrocks assumed, just like
Gottfried Wendelin 1635 (see a letter he wrote to Gassendi, dated
May 1st; in Petri Gassendi Opera Omnia, Volume 6, Lyons, pp
427-429; 1658) that the radii of the planetary spheres were directly
proportional to their distances from the Sun. Horrocks also followed
Wendelin in the belie that all planetary spheres subtended the same
angle at the Sun, this being 28 seconds of arc. Under these
circumstances the 'solar parallax' is 14 seconds of arc (i.e.
3.89x10to the -3 degrees). So for example, the radius of Earth
divided by the Earth-Sun distance is the tangent of 3.89x10 to the
-3 degrees. This then means that the astronomical unit is
14733 times the radius of Earth.
(v) Interestingly if we go back to the important measurement of
76 seconds of arc for the angular diameter of Venus as seen from
Earth, by Horrocks, in 1639 we that things nearly fit:
Venus at transit was 0.277 au (i.e. 1.000 - 0.723 au) away from
Earth and subtended 76 sec arc at Earth. Venus at transit was also
0.723 au away from the Sun and thus subtends an angle sec arc at the
Sun , where a= 76x0.277/0.723= 29. As 29 was very close to 28
you can see why Horrocks did not change his mind. He died
rather suddenly and so the story ends!"
Q. Could you explain why birds don't fall
over when they stand on one leg? I have found my balance to be much
better when I stand on both legs.
I was studying some ducks today and have spent several hours
trying to work out why a duck would tuck one leg up, when it could
put both on the ground - Becky
Winn (a great fan)
A. Good question. Herons also spend quite a lot of time on
one leg, but not, I think, pigeons, rooks, or little birds - robins,
tits, or sparrows.
I don't know the answer, but could it possibly be that these
water birds (a) have big webbed feet and can balance better, or (b)
like to get one foot dry occasionally.
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