Helen - It’s a great question.  Portuguese man o’ war, a type of jellyfish but actually they’re lots of different individual animals that live together.  They’re not really considered as being a single organisms which  is rather intriguing.  They can’t actually swim.  Other jellyfish have a way of pulsing their tentacles and moving so they’re basically at the mercy of wind and currents and what’s going on around them.  They have this lovely blue and pink tinged pneumatophore full of gas that floats them on the surface.  That catches the wind if the wind is blowing in that direction. The really cunning thing about these guys is that, like humans, Portuguese man o’ war can be left-handed or right-handed.  About half of them in the population has this pneumatophore that’s shaped towards the left and half that’s shaped towards the right which means they will go in different directions if the wind blows in a certain direction.  I think that’s very cool indeed.  You can imagine why that might be a good thing.  It means that half the population will get swept up on a beach if the wind’s going in a certain direction.  Some of them won’t.  They won’t all necessarily land up on a beach ‘cos that’s not great.  They usually get trodden on and will desiccate very quickly and dry out.  It’s all very much at the mercy of the elements.

Chris - I saw them in Australia, some of them doing exactly as you said. They were getting blown up on the beach because they have this sail on the top which pushes them along.

Dave - There’s a couple of things. The first one is that pure water is quite a good insulator. My girlfriend’s dad used to work in a lab with huge amounts of very high voltages and very pure water. If water’s pure it doesn’t collect electricity very well. You need some salts in it.

Chris - Why doesn’t it?

Dave - Basically, if you conduct electricity in a liquid you want some free ions. You want some positively-charged things, negatively-charged things. If you’ve got some salt in there the salt splits up into negative chloride ions and positive sodium ions. The positive ones move to the negative end of the battery and vice-versa. You get a current flowing. That’s one thing and I think you do lose quite a lot of the power through things shorting out. I think you get some gentle losses through there all the time. If there’s anywhere where it does get a lot of power dumped: if you get a particularly conductive bit of snow or ice it’s going to get a lot of power going through it. It’s going to get hot and it’s going to melt and move away from the rail. It immediately breaks that short-circuit.

Chris - Is it worth bearing in mind that the voltage on things  like the London Underground is a lot lower than  when you have trains doing long distances when it’s much more economically sensible to use high voltages?

Dave - The rails on the ground are about 600V and the overhead lines are about 25,000Vbecause of this effect. If you had a 25,000V on a rail you would get sparks going to the ground. You’d also kill a lot of people! It’s not actually lethal unless you touch it. I think they probably do lose a lot of power through them but less than you’d expect where if you do get a short it’s going to burn out.

Chris - On a foggy day if you go near to a pylon of high-tension cables where higher volts are used you can hear them buzzing as the mist is settling on the insulators supporting the cables. You get the effect you’re describing where the little bit of arcing vaporises the dampness there and that goes away  and then some more settles.

Dave - Yes, if it vapourises it’s going to expand the air out.

Chris - It’s making me cringe just thinking about it.  This is quite a commonly asked question.  The answer is psychologists and scientists don’t know for sure.  One very plausible theory is that when you run fingers down blackboards the frequencies you hear are very high-pitched and the high-pitched frequencies are very similar to the frequencies that animals produce when they’re in distress.  On argument is that we are genetically programmed and tuned-in to be sensitive to those frequencies.  That may alert us that an animal of our own species, perhaps, is in distress.  Perhaps it’s being attacked or eaten or is in danger.  Therefore, by galvanising your attention and waking up and paying attention to that noise you’re therefore on high alert and you can make plans to run away or fight.  That’s probably the best explanation there is for that at the moment.

Helen - I like the idea of experimentation on other animals to see which ones are sensitive to nails on blackboards.  We can put them in rooms perhaps and see which ones jump the most when we scratch our nails.

Dave - The way they do it now is basically that if you’ve got something that’s orbiting the moon its weight is going to produce gravity. The stronger the gravity the farther you can orbit. As soon as you put a satellite orbiting the moon you can measure its mass quite accurately. By looking at how a satellite’s orbit changes as it goes around the moon you can see tiny variances in mass from mountains and things. You can get very accurate gravitational maps of the Earth and find things like ore bodies. If you’re got very dense rock somewhere then that’s going to pull you down a bit. In the distant past you might have worked it out the size of the tides on the Earth because you know how far away the moon is and how string gravity is on the Earth. By working out how much of an effect the moon has on the water you might get some idea of how much mass must be in the moon.

Chris - You also know that the tides used to be much bigger than they are today because the moon and the Earth used to be much closer together. There are fossilised tidemarks that geologists have uncovered that are metres in height. Because the moon has slowly migrated away from the Earth because it is moving away from us by about 3cm a year. You end up with progressively smaller and smaller tides. We’re now down to the more reassuring several metres rather than the 100s of metres that they were getting at one time.

Dave - That effect is because the tides are slowing down the Earth and also speeding up the moon. You can actually detect the slowing down of the earth because 200million years ago you could tell how many days were in a year. The corals grow a little line every day in a year. You can see the range in a year of maybe 400 days in a year about 200 million years ago.

Helen - That’s a great question.  These wonderful two metre long creatures are giant tube worms that live in the middle of the ocean, very deep down in the deep dark ocean where there’s no connection to the light and they only survive because they have this symbiotic bacteria that harness chemicals that live inside them. How do they move from one vent to another? They do live in very clustered environments a long way apart from each other. There’ve been a couple of different studies that look at the genetics and first of all they found out how long their larvae can live for. One theory is that they have eggs and sperm. We can see them fertilising externally, outside of the worm. They form larvae and in the laboratory those have lived for 38 days. The idea is that that’s enough time for them to hitch a ride on a plume of water. We know there are these nutrient buoyant plumes of a mixture of hot and cold water very deep down, we’re talking km down in the middle of the Pacific Ocean. That’s enough time for them to drift and find another vent for them to live on. These are also very short-lived things. These hydrothermal vents come and go as changes in the sea floor take place. Really they’re quite ephemeral and that’s one thing that they’ve done. Genetics are likely to be what happens. You’ve got very distinct populations that are fed by just a few larvae arriving and starting a new population as new vents open up black smokers and things like that. We’re talking 400 degrees centigrade. Crazy ecosystems.

Dave - That’s a good question. Yes, all the salt is going to go down to the water table eventually, it’s going to dissolve in water and run down where water is. It’s either going to run down the streams and down into rivers and out into the ocean where it’s not going to make much difference. The stuff which goes into the water table will make some difference. Locally it could be an issue. We are expecting when you put the salt down it will have a significant effect on the degree of saltiness in the ditches down the side of the road. Overall it’s not going to be very big an effect just because of how big the UK is. 1mm of rain over a square kilometre is the equivalent of a thousand tonnes of water falling on the UK. The UK’s 40,000 square kilometres so just one millimetre of rain, less than a 20th of an inch is going to be equivalent to 40 million tonnes of water.

Chris - Although to be honest that’s not all falling evenly and uniformly like that. There could be some areas that could end up with local salt build-up but it’s trivial in the grand scheme of things.

Dave - If it’s in a big enough area there’s going to be enough rain to dilute it down where it’s not going to be a problem to drink.

Chris - This is quite simple and it comes down to the fact that we have a hydrological cycle. The sun put energy into the Earth. Each square metre of the Earth’s surface, on average, gets energy at the rate of about 1kW from the sun. This energy goes into the sea water and the water molecules gain enough energy sometimes to evaporate. So you have water vapour which leaves the sea and goes up to form clouds. Those clouds then travel over to land. When they’re forced to rise over things like high mountains then in order to rain they have to rise. They lose some mass in the form of water precipitation. That fresh water comes down out of the clouds and lands on the ground. It goes into rivers and streams and picks up tiny amounts of minerals and salts which it dissolves on its way, percolating through the ground. There’s not that much there so the water tastes fresh. You can just detect trace amounts of these chemicals in the river water and in pond water. As it makes its way down towards the sea it then takes with it those salts. When the water then re-evaporates in the ocean that’s just fresh water evaporating. The salts get left behind. Over millions of years you then accumulate salts in the sea until you see the salinity that you see today. The sea isn’t actually going to get much saltier because once you get to a certain threshold concentration you start to get other chemical reactions kicking in which limit the accumulation further of any more dissolved ions or salt. As a result it just contains at the level it’s at.

Dave - Actually the way we get most of our salt from is edges of the sea: very ancient shallow seas where there’s lots of evaporation at the edge of a desert. The sea keeps flowing in, lots of water evaporates and the salt crystallises out. That often gets buried – there’s huge amounts under the North Sea. There’s quite a lot in Cheshire.

Chris - And you can go to Poland and some very famous salt mines which were these salt pans, weren’t they. Absolutely amazing.

Chris - Did you ever do the thing when you were at school where you put your metal pencil sharpener in your mouth?

Helen - I’ve never tried!

Chris - Certain pens that have a clip that you put over your pocket and it’s made of metal, a different metal to the pen. You suck those and occasionally you notice a tingly sensation in your mouth. Have you ever noticed that?

Helen - I’m going to try now but my pen‘s plastic.

Chris - That won’t work. The point is that when you mix two different metals together and you have an electrical conductor between them and you have an electrolyte – saliva has lots of salt in it so it’s an excellent conductor. You can get a chemical reaction happening between the two metals. One metal, the more reactive on will form ions and it will give up electrons which will flow through the electrolytes to the less reactive metals. That’s how ht e reaction occurs. As a result if you tough two metals, in this case you’re touching with a filling – that’s mercury-silver amalgam – the aluminium will dissolve to make some aluminium ions. It’ll make some electrical current which you will experience in your mouth as this tingling sensation and it will also react with the saliva to produce some hydrogen and some oxygen gas perhaps on the two different surfaces. You will actually deposit some material on your filling. You’re basically turning your mouth into a battery.