Answers to last issue's Do You Know?

1. What is dark matter?
Ans: See the article in this issue of JM.
2. How can some one control a machine just through thoughts?
Ans: When a person has the misfortune to lose a limb (arm or leg), they are fitted out with an artifical limb in its place. Nowadays, these artifical replacements, called prosthetics or prosthetic limbs, can be controlled with sensors so that they can actually be used for doing work.
The Rehabilitation Institute of Chicago in the US has introduced the first woman to be fitted with its "bionic arm" technology. Claudia Mitchell had her left arm amputated at the shoulder after a motorcycle accident. She can now grab a drawer and pull it with her prosthetic hand by thinking, "grab drawer pull."
This demonstrates that a person can successfully control multiple, complex movements of a prosthetic limb with just his or her thoughts. It opens up a world of possibility for amputees.
The setup -- both surgical and technological -- that makes this feat possible is almost as amazing as the results of the procedure.
The "bionic arm" technology is possible mainly because of two facts of amputation. First is a property of the area in the brain called the motor cortex that controls voluntary muscle movements. Voluntary movements are those that are made deliberately, such as picking up a cup of tea. Involuntary actions are those that "can't be helped", like sneezing or coughing.
Voluntary actions are possible because the brain signals the relevant muscles to perform a particular job. It turns out that this area in the brain still sends out control signals even if the limb is cut off, that is, even when the muscles are no longer available to be controlled.
The second fact is that, when doctors amputate a limb, they don't remove all of the nerves that once carried signals to that limb. So if a person's arm is gone, there are working nerve stubs that end in the shoulder and simply have nowhere to send their information.
If those nerve endings can be commanded by the brain to "grab handle with hand", for instance, they are still capable of transmitting that signal even though the muscles they are supposed to command are no longer there. 
What doctors have done is to attach these nerve endings to another set of working muscles that still exist. This is like re-routing the signal.
For example, in the bionic arm, the nerve endings that control the missing hand are now attached to a set of chest muscles. It takes several months for the nerves to grow into the chest muscles and become a part of it.
Now comes the amazing controlling mechanism: electrodes are placed on the surface of the chest muscles. These electrodes are used to control the movement of the prosthetic arm, one electrode for each movement (for different joints and movements possible in the arm).
So what happens is this: the person's brain gives the command to "open hand". This command reaches the nerve ends which earlier ended in the cut-off limb, but are now relocated in the chest. The command causes the chest muscle to contract. This results in a signal to the electrode that sits on the muscle. The electrode sends a command to the motor that controls the bionic hand, asking it to open.
Since each nerve ending is attached to a different piece of chest muscle, a person wearing the bionic arm can move all six motors simultaneously, resulting in a pretty natural range of motions for the prosthesis. So the artificial hand becomes as useful and capable as the natural one.
3. How strong are the magnets in an MRI machine? Should you worry about the loose change in your pocket?
Ans: Magnetic resonance imaging (MRI) is used to visualize detailed internal structures inside the body. It is used for medical diagnosis. It provides much greater contrast between the different soft tissues of the body than computerised tomography (CT) does. This makes it useful in imaging the brain, heart, muscle and skeletal tissues and for cancer imaging.
Unlike CT, it uses no ionising radiation, but uses a powerful magnetic field. This field usually acts on the hydrogen in water molecules in our body (we have lots of water in all our tissues). The magnetic field aligns the nuclear magnetisation of hydrogen atoms and orients it.
The hydrogen nuclei then produce a rotating magnetic field detectable by the scanner. This signal can be manipulated by additional magnetic fields to build up enough information to construct an image of the body.
The biggest and most important component in an MRI system is the magnet. The magnet in an MRI system is rated using a unit of measure known as a Tesla. Another unit of measure commonly used with magnets is the gauss (1 Tesla = 10,000 gauss). The magnets in use today in MRI are in the 0.5 Tesla to 3.0 Tesla range, or 5,000 to 30,000 gauss. Compared with the Earth's magnetic field of half a gauss, this is really very large indeed.
The magnetic force exerted on an object increases exponentially as it nears the magnet. Imagine standing 5 m away from the magnet with a large metal scissors in your hand. You might feel a slight pull.
Take a couple of steps closer and that pull is much stronger. When you get to within 1 meter of the magnet, the scissors is likely to be pulled from your grasp. The more mass an object has, the more dangerous it can be -- the force with which it is attracted to the magnet is much stronger. Mop buckets, vacuum cleaners, IV poles, oxygen tanks, patient stretchers, heart monitors and countless other objects have all been pulled into the magnetic fields of MRI machines. Smaller objects can usually be pulled free of the magnet by hand. Large ones may have to be pulled away with a winch, or the magnetic field may even have to be shut down.
Because of the power of these magnets, the MRI scanning room can be a very dangerous place if strict precautions are not observed. Metal objects can become dangerous projectiles if they are taken into the scan room.  For example, watches, paperclips, pens, keys, scissors, and any other small objects can be pulled out of pockets and off the body without warning, at which point they fly toward the opening of the magnet at very high speeds. Since the patient is placed there, he or she can be injured, as well as others in the room.
Credit cards, bank cards and anything else with magnetic encoding will be erased by most MRI systems.
Because of this, patients are carefully checked to be sure they have no metal objects while entering an MRI room. Heart patients with pacemakers cannot be scanned or even go near the scanner! 
4. My father bought me a "glow-in-the-dark" mickey mouse. How does it work?
Ans: You see glow-in-the-dark stuff in all kinds of places, but it is most common in toys -- balls, yo-yo's, games.
If you have ever seen any of these products, you know that they all have to be "charged". You hold them up to a light, and then take them to a dark place. In the dark they will glow for 10 minutes to several hours. Usually it is a soft green or yellow light, and it is not very bright. You need to be in nearly complete darkness to notice it.
All glow-in-the-dark products contain phosphors. A phosphor is a substance that radiates visible light much after being energized.
A fluorescent light, for example, has a mixture of phosphors that together create light that looks white to us. Here the light is radiated immediately after it is absorbed, so that the light turns off when the switch goes off.
In the case of phosphorescence, the energy absorbed during exposure to the light is released at a low intensity over a long time, even after the source of energy is switched off.
To make a glow-in-the-dark toy, you need a phosphor that is energized by normal light and that has a very long persistence, that is, it lasts a long time.
Chemists have created thousands of chemical substances that behave like a phosphor. Two phosphors that have these properties are zinc sulphide and strontium aluminate. Strontium aluminate is newer -- it's what you see in the "super" glow-in-the-dark toys. It has a much longer persistence than zinc sulphide does. The phosphor is mixed into a plastic and moulded to make most glow-in-the-dark stuff.
Some "glow sticks" glow due to a chemi-luminescent process which is not phosphorescence. Here a chemical reaction causes the glow. When such a reaction happens in a biological organism it is called bio-luminescence.  For instance, glow-worms, fireflies, and some types of fungus, such as mushrooms, show this form of luminescence.