Gene combo causes susceptibility to tuberculosis

This week it looks like researchers have discovered a key reason why some people are susceptible to TB (tuberculosis) and others aren’t. Publishing in the journal Cell, Lalita Ramakrishnan and colleagues from the University of Washington think that it’s the levels of an enzyme called LTA4H which give some people better immunity.

And it’s not those with more of the enzyme who are the winners, nor people with the lowest levels of LTA4H. Similar to Goldilocks and the three bears, it’s actually those individuals who have a middling or ‘just right’ amount of the immune enzyme who have TB resistance.

People who are heterozygous or have two different versions of the gene which makes LTA4H have this middling amount of the enzyme. The researchers tested this in a controlled environment by looking at zebrafish which had been selected to produce different levels of the enzyme.  It soon became apparent that LTA4H was playing some role in their immunity. Ramakrishnan then compared her findings with human geneticists from Washington, Vietnam and Nepal to see if they were the same. And it emerged that it was this heterozygosity in people which produced the ideal levels of the enzyme.

What’s interesting about this is that, for a long time it’s been known that people with a nasty case of TB may improve if you give them a dose of anti-inflammatories. It may be that these people are producing too much of LTA4H and by giving them anti-inflammatories you reduce the effect of the enzyme to a medium level; this makes life difficult for the TB bacteria - making the patient feel better.

And it’s an important finding because there are now many strains of TB which are drug-resistant. If you can tinker with the human immune system instead you might come up with a better solution to the problem. Plus there’s the added bonus that LTA4H also confers immunity to other mycobacterial infections like leprosy.

7th Mar 2010

Discuss in Forum

East Siberian Methane Timebomb

Scientists have discovered that millions of tonnes of sequestered methane locked up beneath the shallow ocean shoreline of Siberia are becoming unstable and escaping into the air, threatening to accelerate climate change.

The findings, which are published in the journal Science by University of Alaska Fairbanks researcher Natalia Shakova and her colleagues, suggest that the East Siberian Arctic Shelf, which covers an area of about 2 million square kilometres, is releasing more methane than the rest of the world's oceans put together.

But unlike other methane sources, where the gas is produced by bacteria breaking down organic matter in sediments, the Siberian methane deposits are pre-made and sequestered under the sea as so called hydrates - gas-filled watery cages.

This means that any event that destabilises the deposits can cause them to release their stored methane.

"We think that's what's happening here," says Shakova. "Warming temperatures, and the ingress of the ocean as sea-level rises, are causing the methane release to accelerate, possibly precipitiously."

The team made the discovery by mounting a series of expeditions to collect water and air samples from the Siberian coastline. What they found were very high concentrations of the gas in the seawater, arguing for an undersea source.

The concern is that, with global warming, more fresh water entering the area from melting permafrost will cause local sea temperatures to rise, further destabilising the methane deposits and causing the seafloor to surrender its stored gas in a series of convulsive oceanic belches.

This would dramatically accelerate the rate of global warming because methane is at least 30 fold more potent as a greenhouse gas than the equivalent amount of CO2.

"The release to the atmosphere of only one percent of the methane assumed to be stored might alter the current atmospheric burden of methane up to 3 to 4 times," says Shakova. "The climate consequences would be hard to predict..."

7th Mar 2010

The Earliest Symbolic Scratchings

This week archaeologists have described the discovery of some of the earliest evidence for advanced human thought. Publishing in the journal PNAS, Pierre-Jean Texier and colleagues have analysed nearly 300 bits of carved ostrich shell from a site in South Africa.

These shell fragments are thought to be about 65,000 years old and were found at the site of Diepkloof rock shelter where there are layers and layers of Middle Stone Age archaeology. These eggshell fragments have a fairly common motif on them which involves the scoring of two parallel lines and some cross-hatching linking them. So they look a bit like a simplified picket fence drawing.

Doesn’t sound like a very hard graphic to achieve – could it just be someone doodling? Well the authors of the study think not because the design was prevalent on so many pieces. Also when they tried to copy the design by using knapped flints and a new ostrich egg they found it was really tough to score any sorts of lines on the surface. So this work does seem to be very intentional.

It’s not the first evidence of artwork per se as there have been shell bead discoveries, also from Africa of about 75,000 years old and some more examples farther away this time, in Isreal, which date back 90,000 years. But these ostrich shells are one of the earliest examples of graphic design.

It’s important because it demonstrates what’s known as symbolic thought; the idea that some kind of decoration or image can carry a meaning that is understood by other members of your society. So for example, if you have decoration on your ostrich egg water vessel – it might imply you belong to a particular group of people, maybe you’re making yourself more attractive by doing it, maybe it was something only the adults or only the women did. We may never know what these graphics mean but they do point to some sort of advance in human thought about 65,000 years ago.

5th Mar 2010

Discuss in Forum

Researchers wind back Earth's Magnetic Clock

Scientists have found evidence for a magnetic field around the Earth going back at least 3.5 billion years, 250 million years earlier than previously thought.

The results also reveal that, at 50-70% of present-day levels, the field would have been much weaker than it is now, meaning that the early Earth would have taken a harsh battering from the solar wind, stripping away water and light elements like hydrogen.

The discovery, which is published in Science, was made by University of Rochester researcher John Tarduno and his colleagues. He and his team scoured South Africa for ancient quartz rock samples dating back more than 3.5 billion years. These were tested using a device called a SQUID (superconducting quantum interface device) magnetometer, which can read the magnetic field imprinted into rocks when they first form and being to cool.

In this way the Earth's magnetic field can be reconstructed going back billions of years. Previously, similar approaches had yielded evidence for a magnetic field around the planet from about 3.2 billion years ago but the new results push this date back to 3.5 billion years, indicating that the "geodynamo" - the pool of circulating iron at the Earth's core - was active by this time.

Critically, the results also show that the magnetic field was only half as strong as it is today. This, coupled with the fact that the young Sun was ejecting a solar wind far fiercer than it is today, would have meant that the planet had a harder time fending off the onslaught, which would have robbed the Earth of large amounts of water and would probably have meant the northern lights would have been visible from London rather than northern Norway!

7th Mar 2010

Magnets Mitigate Migraine

Chris -   Also in the news this week, researchers have shown that you can knock migraines on the head with a magnet.  Dr. Richard Lipton is a neurologist at the Albert Einstein College of Medicine.  He’s based in New York and he is with us now.  Hello, Richard.

Richard -   Hi, there.

Chris -   Welcome to the Naked Scientists.  You’ve worked on migraines but many people may not understand exactly what a migraine is.  So could you first of all tell us what a migraine really is, medically speaking?

Richard -   Sure.  So migraine is a specific headache disorder that’s characterised by pain that’s usually one-sided, usually throbbing, often associated with a visual display called an aura, but always associated with something other than just pain, sometimes aura, sometimes nausea, sometimes sensitivity to light or sound.

Chris -   And what do we think is going on in the brain to trigger these, initially, visual effects and then this throbbing pulsing headache that makes people do other things like retire into a dark room and sometimes unfortunately experience nausea?

Richard -   Well for migraine with aura, which was what my study was about, there’s a lot of evidence that what’s going on in the brain is an event called cortical spreading depression.  And in cortical spreading depression, which you can also produce very easily in experimental animals, you have a wave of excitation followed by a wave of inhibition marches slowly forward over the surface of the brain.  As it marches, if it’s in visual cortex, it produces spots of light, the zigzag lines, and then the inhibition produces a greying out of vision which is sometimes called the scotoma.

Chris -   And then people get the pain, but why do they experience pain?

Richard -   The link between aura and pain probably has to do with the fact that there are pain sensitive fibres in the membranes that surround the brain, referred to as the meninges, and the aura itself directly activates these pain sensitive fibres in the brain which are parts of a nerve called the trigeminal nerve and that is likely how aura initiates pain in migraine.

Chris -   So in your study, you were asking, can a pulse of magnetism alter the outcome of someone seeing initially these auras.  Does it prevent them going on to get a headache?

Richard -   Well, the method we used is called transcranial magnetic stimulation.  It’s a method that’s been around for 30 years.  The idea is that if you apply a powerful magnet to the surface of the skull, the magnetic field penetrates through the skull into the brain and induces a small amount of current flow and depending on where you do it and when you do it, that can have either diagnostic or therapeutic applications.

Chris -   So how many people did you enrol in your study and what were the outcomes?

Richard -   So here, the idea was to use a magnetic pulse to induce a current during the aura of migraine with the idea that if you induce the current flow, you would disrupt that march of electrical activity and possibly prevent or dramatically reduce pain.  So we ended up randomizing about 200 people, 160 of whom we ended up treating either with the real magnetic device or with a sham device that vibrated and clicked, but did not deliver a magnetic field.  And we found that of the people who got the real device, 40% were pain-free 2 hours later and remained pain-free at 24 and 48 hours, so most of them.  Whereas only 20% did that well when they were treated with the sham device and that’s a result comparable to what you see with the best as available medical therapy.

Chris -   Speaking of which, I’ve got an email here from Seraphina Anderson who says, “Why don't normal pain killers like paracetamol or ibuprofen work when you get a migraine?”

Richard -   Well sometimes they do and you know, and like every condition, there’s a broad spectrum of severity.  So, ibuprofen and paracetamol may work if you treat the migraine very early.  If you wait and wonder if you need to treat, oral medications become less effective because migraine also affects the gut and you may not absorb the medication as well.  For people who don't do well with over-the-counter medications, there’s certainly a wide range of prescription drug options that are very effective.  So I’m certainly not saying this is the only way to treat migraine.

Chris -   Given how common it is, very large numbers of people suffer with migraines, is your method safe, to your knowledge, and therefore, what’s the next step?  Will we be seeing magnetic stimulators on the shelves of pharmacy shops so people can go and get one if they regularly suffer migraines?

Richard -   My hope is that the answer is yes.  So in the UK, 18% of women, 6% of men have migraine so it’s an extraordinarily common disorder in the UK, in the US and Western Europe or really around the world.  Yes, the hope is that this will receive regulatory approval as a medical device and that it will become available to people who want to use it.  There is a portable device -  For most of its 30-year history, TMS was given with a large 70-pound device that cost perhaps $25 to $30,000 that was kept in doctor’s offices and used by medical personnel.  We studied a portable device that weighs about 3 pounds.  It’s about the size of a hair dryer and the intention is that people will take the device home and when they get a headache, they’ll have an alternative to reaching for either an over-the-counter or prescription medication.

Chris -   Let’s hope so.  Thank you very much.  Richard Lipton, who’s at Albert Einstein College of Medicine and that research, if you want to read up a little bit more about it, it’s in the April edition of the Lancet Neurology.

March 2010

Capturing Sunlight on a Nano Scale

Diana -   Solar cells aren’t a new concept, but they are an area of massive growth.  In fact, photovoltaics as they're known are thought to be the fastest growing energy technology.  But great as solar sounds, it’s long been held back by the relatively poor efficiency of the cells themselves which is what researchers are trying to improve.  Niraj Lal is a researcher from the NanoPhotonics group at Cambridge University’s Cavendish Laboratory.  Hi, Niraj.

Niraj -   Good day.

Diana -   So, can you tell us a little bit about why current cells are poor performers?

Niraj -   I think the reason why you don't see solar cells on roofs everywhere you go is because they're expensive to make.  The materials that they are made out of are pretty expensive and also, they're not as efficient as we’d want them to be.  So, when sunlight hits a solar cell, some of it gets reflected off the top of the solar cell and some of it goes straight through and then out and back again without even being absorbed.

Diana -   So how can we use nanotechnology to try and improve that?

Niraj -   Scientists all around the world are finding that when we structure a material on the nanoscale, a really small scale, it interacts with light in different ways - ways that we haven’t seen before.  And one of those ways is to concentrate light in regions that we haven’t been able to concentrate light in before - on the top of a metal surface.  That’s what’s known as plasmons and we can use these concentrated bits of light to increase absorption in the solar cell and increase efficiency.

Diana -   But how does that actually work?

Niraj -   For certain metals - typically gold and silver or sometimes with copper and aluminium you get the same thing - when you shine light on them, if you shine light with just the right colour, and you have the structure just right, you can set up a resonance inside the metal, inside the charges of the metal.  So if you shine light on a metal surface and you excite these charges going backwards and forwards, you can setup a concentrated bit of light along the top surface of the metal, and that’s what’s known as a plasmon.  That’s what we’re trying to do.  We’re trying use those plasmons to increase the efficiency of solar cells.

Diana -   And you've got an audible example of this setting up of resonance with you.  Could you give us a demonstration?

Niraj -   Yeah.  So what I've got here is a Buddist singing bowl; monks use it to help them meditate.  It’s a brass bowl, about the same size as a cereal bowl.  What I've got here is a wooden rod and I'm kind of rotating it against the edge of the bowl.  If I drive it with just the right frequency, I can set up a resonance response inside the bowl.  So that’s a mechanical response, a mechanical resonance, but the same thing happens with light for structures about 100,000 times smaller.  That’s what my PhD is about – using structures like this to increase the absorption, increase the efficiency of solar cells.

Diana -   So how do you make these nanostructures then?

Niraj -   It’s pretty cool.  What they do is they get the beans from a bean bag, so polystyrene spheres, but a lot smaller and they put them on a clean surface of a metal in a solution and they dry them.  As they dry, they pack into a hexagonal lattice.  When they're all dry and all set, we grow gold or silver or another metal from behind them using electro-chemistry.  As we grow them we get to about half height or a little bit higher than half height, we stop and then we dissolve away the spheres – dissolve away the bean bags, and we’re left with a field of little bubbles everywhere and that’s what we call nanovoids.  We use their plasmonic resonance to increase or try to increase efficiency of solar cells.

Diana -   And just how much more efficient can they become with this technology?

Niraj -   So last week we made a batch of solar cells that showed about twice as much electricity at certain wavelengths.  It’s a preliminary result and we’re still trying to figure out exactly what’s going on but it’s exciting.

Diana -   So presumably, these haven’t been deployed anywhere yet and no one’s using them across the world?

Niraj -   Not yet.  I think it’ll be a while before anything like this actually gets on top of a roof but efficiencies are increasing every year and I think, if this works, in 5 to 10 years, we’ll see them on roofs.

Diana -   Fantastic!  Well thanks Niraj.  That’s Niraj Lal, he’s based in the NanoPhotonics group at Cambridge University.

March 2010

Solar Power in Southampton's Structures

Diana -   You might think that solar cells work best when you pack in as many of them as possible - but this isn’t always ideal.  We sent Meera Senthilingam out to find out how cleverly designing solar cells into the structure of a building can make them multitask.  So as well as offsetting some of the energy demands, they can also act as a shade to keep down air conditioning costs in the summer and as a roof to keep out the rain.

Meera -   This week, I'm at the Highfield Campus of Southampton University in the George Thomas building which is their student services building.  The reason I'm here and what makes this building so special is that it has a photovoltaic atrium.  This building, as well as a few other buildings here on campus, are powered or partly powered by solar cells.  Here to tell me a bit more about the design of this building and just how much electricity it produces is Patrick James from the sustainable energy research group here at Southampton.  Now Patrick, this photovoltaic atrium is quite impressive.  Tell me a bit more about the design.

Patrick -   We had the original building which was built in the 1960s and we needed to expand the amount of space for offices, so we built a new building adjacent to it.  Because of the tight footprint of the space, it was better to link the two buildings together and therefore the designers went for this approach of an atrium; a linking space, open to all levels.

Meera -   So you set about designing this atrium in a way that would capture some solar energy and therefore contribute to some of the power needed by the building?

Patrick -   Yes, that’s right.  The decision was made to go for the atrium with a normal glazed roof and a shading solution with internal roller blinds; and we were asking the question, “Could we do the same thing with a photovoltaic glazing?”

Meera -   How large are each of these solar cells that are up on the roof and how many of them are there?

Patrick -   Each solar cell as we look up is 125 by 125 millimetres square but these are actually formed together into what we called a laminate which is these big sections of 3 by 2 metres and we have 63 of those with a total active area of around about 200 square metres.  And so what you can see here is a roof where the cells are spaced apart to provide sufficient daylight throughout the year for the space but also provide a shading solution for the summer months so we don't overheat this space.  The primary function is a weatherproof barrier where it must stop the rain coming in to the atrium space, that’s number one, then the cells provide day lighting and solar control, and of course we generate electricity.

Meera -   And how much electricity is generated by this?

Patrick -   The generation from the solar cell depends on the amount of sunlight and the temperature of the cell.  If the cell gets hotter, its output drops slightly.  This array is actually highly optimized because of the fact that it’s south facing, it’s at an ideal elevation, around about 35 degrees, and we also get some reflected light from the roof space in front of the atrium.  So we get an additional albedo effect.  So because they are at a very good alignment and roof pitch, they generate around about 900 kilowatt hours per kilowatt peak installed per year.

Meera -   And what does that translate to for people that aren’t familiar with kilowatt hours?

Patrick -   As a residential customer at home, 1 kilowatt hour is one unit of electricity on your bill and that’s what you pay 12 pence for.  So, 900 lots of 12 pence, so around about 100 pounds per year worth of electricity.  We have 12 kilowatts here, so it’s around about 1,200 pounds a year.

Meera -   How much is the total electricity that this building requires is that?

Patrick -   In terms of the electrical demand of this building and because this is offices, majority of the demand is electrical, this generation of the array is around about 6% of this building’s demand.

Meera -   And so, would you say that’s a good offset?  Is that a good amount to be produced?

Patrick -   Obviously, in the UK, our irradiance levels are lower than for example in Spain.  If we just simply moved this building to southern Spain, we will generate 50% more per year.  This application is really showcasing the fact that if you consider the multifunctionality of elements, that’s where you can get the real benefit.  It’s a shading solution, it’s a day lighting solution, it’s a weatherproof barrier, and it also generates electricity.  And when you consider all these elements together, this solution makes economic sense.

March 2010

Roll Up and Roll Out - Flexible Solar Cells

Chris -   One of the problems with conventional solar panels is that they're very heavy.  They're also fragile and they're stiff and that means it’s very tricky to transport them and to install them.  A flexible solar cell that you could roll up and then readily transport would be an ideal solution.  PowerFilm Solar is an American company and they're doing just this.  They're developing what we call ‘thin film photovoltaics’ and we’re joined now by Dr. Frank Jeffrey and also Mike Coon they’re from PowerFilm Solar and they're going to explain to us how they work.  Frank, hello, welcome to the Naked Scientists.

Frank -   Thank you.

Chris -   Tell us first if you would, how does your architecture, your flexible cells, actually differ from the rigid ones that we see people putting on their roofs?  How do you make them bend?

Frank -   The principle part of the solar cell itself in our cells is amorphous a silicon, which has an extremely high absorption coefficient so that we can have extremely thin semiconducting material that will still absorb a good portion of light.  That thin material, even though if it were thick like a crystalline wafer, it would break, in the same type structure when it’s thin enough becomes flexible and tends to bend rather than break.  So, that’s the key part, our basic absorber layer that absorbs the solar energy is only say, 5,000 angstroms thick.  So it’s quite thin and flexible and we put it on a thin film plastic substrate that is also flexible and adds mechanical support and strength to the solar component.

Chris -   So when you say it’s flexible, how flexible are we talking?  Could you roll this up like a newspaper or would it not tolerate that kind of treatment?

Frank -   Well, if we have the basic substrate and solar material itself, we can roll it up to a diameter of a pencil and it does just fine.  Actually, some applications we do roll that small for storage.  That’s mostly a space type application but normally, we put a heavier encapsulant on the outside to protect against earth’s atmosphere and that means around maybe a 3-inch diameter is what a commercial cell or module that we sell will roll up to comfortably.

Chris -   That’s still pretty impressive to get it down so small.  If we could zoom in with a microscope and just examine the structure of your cells, what would we see?  If you could just paint a picture for us so people can appreciate exactly how they're configured.

Frank -   Okay.  Maybe an electron microscope in order to see it, but we start out with a basic film of polyamide plastic to build it all on.  So that’s the bottom layer that you would see and that may be a 25 to 50-micron thick plastic film.  On top of that, we put a metal layer, principally aluminium, that acts as a back electrical contact.  They're able to carry the electrons off the back surface of the solar module or solar cell.  Then there are six layers of silicon forming actually 2 diodes, a thick diode in the bottom that absorbs the red light and a thinner diode in the top that absorbs the blue light. By having 2 diodes, we get a higher operating voltage and lower current which means we don't lose as much energy in resistance of the leads coming in and out.  Then on top, we have a transparent oxide conductor.  It’s not all that easy to make something that’s both transparent and conductive, but that’s the type of film we use.  That allows the light to come in and also carries the current off the face of the solar cell.  So that’s the stack from top to bottom and then clear plastic, generally a polymer encapsulation both front and back to protect it from moisture and outside weathering, and that type of stuff.

Chris -   Ingenious to manage to have something that absorbs both the red and the blue so that you don't waste any energy.  How much energy do you extract?  If I compared your system head-to-head with one that I could buy off the shelf to put on my roof today, how would the efficiencies compare?

Frank -   If you compare to the different technologies out there, ours is quite a bit less per square foot.  We generate about 5 watts per square foot as opposed to crystalline silicon which is more in a range of 15 watts per square foot.  So, the output is considerably less.  Part of the point of our approach also is very low cost manufacturing so that ultimately, we can be competitive in the cost per watt generated and in specific markets that require us to be lightweight and thin such as integrating it in building panels, we can be competitive with crystalline silicon on a cost basis, and in an application basis.

Chris -   So no such thing as a free lunch.  And Mike Coon, let’s bring you in here.  I suppose the payback must be that you've got very good portability, there must be many applications for something like this which can be rolled up, packaged away, and taken somewhere where you need instant power on demand in the middle of nowhere.

Mike -   That’s right.  On one end of the continuum, we have our products which serve the portable power market especially well because of the lightweight nature of our material and on the other end of the continuum, talking more about our building integrated products, our larger scale products which can be up to 30 feet or approximately 10 metres long for larger scale building integrated applications.  But on the portable area, the light weight is especially important because we can provide power unlike others that can be extremely lightweight, can be portered in, and can be extremely durable.  For example, the US Military has shot holes through it and it continues to perform and that’s because of this printed interconnect which Dr Derrick Grimmer developed early on for the company.

Chris -   Tremendous!  So in other words, you've got something which is quite literally bomb proof.  How are you actually seeking to use this?  Who are your markets?  Who is taking this product and deploying it in the field?

Mike -   Yes.  There are currently three primary market segments that we’re serving and a fourth one which we’re launching and in process of gearing up for.  The existing markets that we’re selling in to are the commercial industrial markets, a variety of applications ranging from providing panels for GPS asset tracking on semi-tractor trailers to remote data collection, electric golf carts, campers, RV panels, the whole gamut.  Also the military market is very important for us.  We’ve developed products which range on the one hand from small AA chargers to 5 to 60 watt portable chargers to charge everything from ruggedised laptops and notebooks, to medical refrigeration, to remote sensors, as well as in our larger 1 to 3 kilowatt power shade products which provide not only a remote portable power, but also the shade benefits that was mentioned earlier.  These are very rugged durable products that can go over existing shelter structures and about four or five man hours can be set up with two to four soldiers.

Chris -   So I guess that if you've got say, a military camp, they're in the middle of nowhere, previously, power had to come from someone carting either very heavy batteries or a big diesel generator and all the fuel for it.  Now, you've got a system where you could deploy this, it looks because it’s flexible, like a tent to all intents and purposes, and it’s going to provide mobile power.

Mike -   That’s right.  It’s very much designed to meet the power needs of today’s war theatre which remote outposts are incredibly important, increasingly important, such as in Afghanistan. We provide energy solutions which can be integrated either independently for targeted use of power as well as integrating part of overall hybrid systems.  One of the important aspects of our technology is it does reduce fuel consumption which can be incredibly costly in those remote areas.  Reducing fuel consumption reduces convoys, which reduces the cost of the fuel as well as the potential risk of casual losses with those fuel convoys.

Chris -   Terrific.  Well thank you very much for joining us to tell us about your work.  That’s Mike Coon and Frank Jeffrey.   They're both from PowerFilm Solar with flexible photovoltaic cells that you can even turn into tents.

March 2010

DIY Photovoltaic Solar cell

Find out how to build your own solar cell, it may not solve the world's energy problem, but it is made from recycled components.

What you need

What to Do

What may Happen

You should find that in full sunlight the diode will produce a significant fraction of a volt and a few µA, but in the dark it doesn't even produce a voltage. You have made a solar cell, though not a very powerful one. Ours produced 4µW - so to power a kettle you would need 500 million of them!

What is going on?

A diode is often used as a one way valve for electricity, a solar cell is just a specially optimised very large diode. So any diode will work as a solar cell, though not very effectively. This is why the diode had to be covered in black plastic (otherwise it would start injecting all sorts of strange voltages into its circuit when the sun came out).

When light

What is a Diode?

A diode is made of semiconductors such as Silicon or Germaniumm these are materials on the edge between an insulator and a conductor. Pure semiconductor doesn't conduct electricity as each atom has 4 electrons which is stable, but if you slightly increase the number of electrons these extra electrons can move around easily and conduct electricity.

They will also conduct electricity if you remove a few electrons. This is because the hole can move around like a sliding block puzzle, as the electrons move in the opposite direction. It is easiest to think of the hole as a virtual positive particle.

A diode is created by connecting a lump of N-type semiconductor to a P-type semiconductor.

If this depletion region is hit by light the energy can be used to knock an electron off an atom creating a free electron and a hole. These are pull in opposite directions and have to flow around the circuit to meet up again doing useful work.

Why is a diode a one way valve for electricity?

If you apply a voltage to the two sides of the diode depending on its direction it will have a very different effect.

Randy Heish sent us these incredible pictures of his own diode-based solar cells - powerful enough to power a clock!

"Guys, thanks for the kitchen science this week: I've been an Engineer for 30 years this August and never realized there's a good reason diodes are encased in black epoxy - circuit design would be a bit more challenging if you had to include light compensation into your design :)
 
Here's my quick verification of your experiment - don't laugh at my soldering job - it was done pretty quickly.  You sold yourself a bit short on the "micropower" limits of these diodes - it is true but still enough power to drive a watch for example:
 
 

The above was taken about 6 inches from a 60W light bulb.  I measured about 1.2vdc from the 4 1N400X diodes in series.
Just so you can see I'm not cheating - no battery!

BTW, I had an article in Circuit Cellar magazine a few years back entitled 'Thermoelectric micropower generation" (issue #113): I used a peltier module with heat sinks and a candle to generate "micro-power" (around 0.5v at 125mA, which I used to power a small AM/FM radio) - quite a bit more power, so this experiment tops the minimal micro-power usage.

Thanks again and keep up the good work - a very informative and thought provoking program."

Written by Dave Ansell