Answers to last issue's Do You Know?

1. How does dialysis work?

A dialysis machine imitates some of the functions of a human kidney. One of the primary jobs of a kidney is to remove waste products such as urea and certain salts from the blood. These are then removed from the body in urine. When the kidney fails in its function, a dialysis machine is needed to perform these tasks.
Without dialysis, all patients with kidney failure would die from the buildup of toxins in the bloodstream. Dialysis makes use of the fact that urea and salt are smaller than the red and white blood cells.
Blood from the patient is diverted into the dialysis machine. A sterile solution made up of water, sugars, etc., is also pumped into the machine. This is called the dialysate. The two fluids (blood and dialysate) never come in direct contact with each other. However, the blood flows in special tubes through the dialysate solution.
The special tubes are made of semi-porous membrane. This membrane allows exchange of urea from the blood to the sterile fluid. However, it does not allow blood cells to pass through. The clean blood is sent back into the patient's body.
This process is repeated after some time when more urea has collected and needs removal.
There is another type of dialysis where the dialysis solution is passed through a silicon tube into the abdominal cavity. The blood is not pumped out of the body: the waste from the blood is again exchanged with the dialysate but within the abdomen. 

2. How much does the Earth weigh? How can you find out?
The answer to that is: approximately 5,974,200,000,000,000,000,000,000 (6E+24) kilograms.
The measurement of the planet's weight is derived from the gravitational attraction that the Earth has for objects near it. 
It turns out that any two masses have a gravitational attraction for one another. If you put two bowling balls near each other, they will attract one another gravitationally. The attraction is extremely slight, but if your instruments are sensitive enough you can measure the gravitational attraction that two bowling balls have on one another. From that measurement, you could determine the mass of the two objects. The same is true for two golf balls, but the attraction is even slighter because the amount of gravitational force depends on mass of the objects.
Newton showed that, for spherical objects, you can make the simplifying assumption that all of the object's mass is concentrated at the center of the sphere. The following equation expresses the gravitational attraction that two spherical objects have on one another:
F = G * M1 * M2 / R2.
Here R is the distance separating the two objects,
G is a constant that is 6.67259x10-11m3/s2 kg,
M1 and M2 are the two masses that are attracting each other, and
F is the force of attraction between them. 
Assume that Earth is one of the masses (M1) and a sphere is the other (M2). The force between them is F = M2 x g, where g = 9.8 kg*m/s2 is the acceleration due to Earth's gravity. When the mass is on the surface of the Earth, R is the Earth's radius (assumed spherical) and the unknown mass M2 cancels in the equation to give
M1 = g R_E^2/G.
The radius of the Earth is 6,378,100 metres. If you plug all of these values in and solve for M1, you can find the mass of the Earth.
It is interesting to note that the gravitational constant was first determined by Henry Cavendish in 1797, more than 200 years ago. He used a torsion balance (see figure) made of a six-foot wooden rod suspended from a wire, with a 2-inch diameter lead sphere attached to each end. Two 12-inch lead balls were located near the smaller balls, about 9 inches away, and held in place with a separate suspension system. The experiment measured the faint gravitational attraction between the small balls and the larger ones.

3. How do escalators work?
Escalators are becoming quite common in Indian cities. An escalator is just a simple variation on the conveyer belt that is widely used in industry. It consists of a series of stairs that are pulled by a pair of rotating chain loops. The stairs move in a constant cycle, moving a lot of people a short distance at a good speed.
While it is exceedingly simple, the system that keeps all the steps moving in perfect synchrony is really quite brilliant.
The essential components of an escalator are a pair of chains looped around two pairs of gears, one at the top and one at the bottom. An electric motor turns the drive gears at the top. These engage with the chain loops, which therefore rotate. The motor and chain system are housed inside the truss, which is a metal structure extending between two floors. See the picture taken from "HowStuffWorks".
Instead of moving on a flat surface, as in a conveyer belt, the chain loops move in a series of steps. The way these steps move forms the novelty of the escalator. As the chains move, the steps always stay level. At the top and bottom of the escalator, the steps collapse on each other, creating a flat platform. This makes it easier to get on and off the escalator.
It is able to do this because each step in the escalator has two sets of wheels, which roll along two separate tracks. The upper set (the wheels near the top of the step) are connected to the rotating chains, and so are pulled by the drive gear at the top of the escalator. The other set of wheels simply glides along its track, following behind the first set. 
Each step has a series of grooves in it, so it will fit together with the steps behind it and in front of it as it flattens out when it reaches the top or bottom.
For safety, the electric motor rotates the handrails along with the chain loops. Both move at the same speed so that the person standing on the moving steps and holding the moving handrail feels stable.

4. Why should you remove your earrings before taking an MRI?
Magnetic Resonance Imaging (MRI) scans help to diagnose different health problems such as multiple sclerosis, brain tumors, torn ligaments, tendonitis, cancer and strokes, to name just a few. An MRI scan is the best way to see inside the human body without cutting it open.
The biggest and most important component of an MRI system is the magnet. There is a horizontal tube -- the same one the patient enters -- running through the magnet from front to back. This tube is known as the bore. But this isn't just any magnet -- it's an incredibly strong system, one capable of producing a large, stable magnetic field.
The strength of a 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 systems create a magnetic field of 0.5- 0.2 tesla, or 5,000 to 20,000 gauss. In comparison to the Earth's magnetic field of 0.5 gauss, these are very powerful indeed.
In addition there are other snaller magnets in the machine. The variable magnetic field is used to record images of the internal organs of the body.
However, the MRI suite 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, paperclips, pens, keys, scissors, jewelry, stethoscopes 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. Credit cards or anything else with magnetic encoding will be erased.
Big objects pose a risk, too -- mop buckets, vacuum cleaners, IV poles, patient stretchers, heart monitors and countless other objects have all been pulled into the magnetic fields of the MRI.
Often, patients have implants inside them that make it very dangerous for them to be in the presence of a strong magnetic field. These include metallic dental implants and even pace makers. Even if the objects are non-magnetic, some surgical implants like artificial joints can cause distortion of the image, leading to a wrong diagnosis.
To ensure safety, patients and support staff should be thoroughly screened for metal objects prior to entering the scan room. So make sure you leave those metal earrings behind!