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570. Who invented the telephone dial or rotary portion of the telephone? — B, R, B, D, and S at Northlake Elementary, Richardson, TX
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Let me start a little earlier, with the automatic telephone exchange: This exchange was invented in 1892 by Alman B. Strowger, an undertaker from Kansas, who first installed it in La Poste, Indiana. The system used electromagnetic relays to recognized a series of pulses and to make the appropriate connections between telephones. While the "Strowger system" remained in use until the advent of modern electronic switching systems, it was improved many times. The pulses that controlled this system were originally made with push buttons and one of the most important improvements was to replace the push buttons with a rotary dial that created the pulses automatically. However, the rotary dial wasn't so much invented as developed and I haven't found any record of the individuals who contributed to that development. No doubt it's a patented device and the patent record probably includes the names of the people responsible. If I can find that patent, I'll add it here.

588. How does a telephone switching system work? Why was it so hard to trace telephone calls? In movies we see people pulling wires in order to trace the origin of a call. - AZ
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Before the advent of electronic telephone switching systems, the automatic switching was done by electromechanical relays. These remarkable devices were essentially 10-position rotary switches that were turned by a series of electric pulses—the same pulses that were produced by the rotary dial of a telephone. When you dialed a "5", your telephone produced a series of 5 brief pulses of electric current and one of these relays advanced 5 positions before stopping. Each number that you dialed affected a different relay so that your called was routed through one relay for each digit in the number that you called. To trace a called, someone had to follow the wires from relay to relay in order to determine what position each relay was in. From those positions, they could determine what number had been dialed. The first few digits of the telephone number determine which exchange (which local switching system) was being called, so those first relays were located in the caller's telephone exchange building. The last few digits determine which number in the answerer's exchange was being called, so those relays were located in the answerer's telephone exchange. As you can imagine, finding your way through all those relays and wires in at least two different buildings was quite a job.

589. If E=mc2 and we know light exists, why is it that light doesn't have infinite mass and consequently why aren't we all squashed? - M
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The equation that you present is a simplification of the full relationship between energy, mass, momentum, and the speed of light, and is really only appropriate for stationary massive particles. In it, E is the particle's energy, m is the particle's rest mass, and c is the speed of light. Since light has no rest mass, the previous equation is simply not applicable to it. I should note that this equation is sometimes used to describe moving massive particles, in which case the m is allowed to increase to reflect the increasing energy of the moving particle. But the use of this equation for moving particles and the redefinition of mass as something other than rest mass often leads to confusion.

A better way to deal with moving particles, particularly massless particles, is to incorporate momentum into the problem. The full equation, correct for any particle, is E2=m2c4+p2c2. In this equation, E is energy, m is the rest mass of the particle (if any), p is the momentum of the particle (if any), and c is the speed of light. While light has no rest mass, it does have momentum and it's this momentum that gives light an energy. Light travels along at the speed of light with a finite momentum and a finite energy. On the other hand, the momentum of a massive particle increases without limit as the particle approaches the speed of light and so does the particle's energy. Thus massive particles can't ever reach the speed of light.


596. How does a sewing machine work? — RD, APO
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A sewing machine uses a spinning shaft to push a needle up and down through fabric. The rod that controls the needle's height is attached to the spinning shaft away from the shaft's axis of rotation so that as the shaft spins, the rod and needle move up and down. This motion resembles that of a child on a tricycle: as the front wheel turns, the child's legs move up and down.

Thread from a spool held above the fabric passes through an eye in the needle's tip, so that as the needle pierces the fabric, it carries the thread with it. A device beneath the fabric catches hold of this thread and pulls it rapidly around a smaller spool of thread (the bobbin). The thread from above the fabric thus fully encircles the thread from this bobbin and the two threads become permanently locked together. When the needle withdraws from the fabric, some of the thread that it carries remains behind, locked around the thread from the bobbin below. With each stroke of the needle, a new joint is created between the thread from above the fabric and the thread from below the fabric. If there are several pieces of fabric lying on top of one another, these pieces become locked together by the intertwined threads.


599. How do motion detectors work? — MK, Port St. Joe, FL
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According to Gabriel Lombardi of Torrance, CA, most home motion detectors use infrared light to sense motion. Moving objects change the amount of infrared light striking a detector at the focus of an array of fresnel lenses. He points out that you can see this array on the front of many motion sensors. Such devices are known as passive infrared or PIR detectors. The motion detectors used in automatic door openers, such as those at the supermarket, usually use radio frequency electromagnetic waves to detect motion.

610. Is magic a real possibility? — LM, Dartmouth, Nova Scotia
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I would define magic as any phenomenon that can't be explained by the normal laws of nature. In that case, I'm afraid that it isn't a possibility. Like most physicists, I'm convinced that the laws of physics can ultimately explain everything that we observe. Violations in those laws would have such terrible complications that even a single "magic" event just can't occur. No doubt, there are people who believe in magic and that view physicists as just another group with a different and incorrect opinion about the world. That's just wishful thinking. Physics has been extraordinarily successful at explaining how the world works. Unlike magic, physics has an internal consistency that is astonishing and it has the ability to predict behavior with enormous accuracy.

615. Is the fact that the small magnetic fields generated by appliances change due to the alternating electric current the reason that EMFs may cause health problems? — MC, Independence, KS
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I believe that the alternating nature of the electromagnetic fields around appliances is at least part of the reason they're suspected of causing health problems. Since these fields are created by an electric current that alternates in direction, they alternate in direction, too. However, I have not seen any credible evidence for there being a relationship between these appliance-related fields and health problems, nor have I heard any sensible physical theory for such a possibility. On the contrary, I have read a number of compelling arguments for why the tiny electromagnetic fields around appliances should have no biological effects at all. I think that the worries about EMFs are unfounded.

617. Does the pull of the moon have any effect on a person's behavior? — PSC, Summerville, WV
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No, but for an interesting reason. While the moon's gravity acts on people, it also acts on everything around them and everything falls toward the moon at the same rate. Because of this uniform falling, we don't feel the moon's gravity at all. This effect is identical to the one that astronauts feel as they orbit the earth—the earth's gravity pulls on them and on their spaceship, but they are falling freely under the influence of that gravity and they don't feel it—they feel weightless. Since we are falling freely under the influence of the moon's gravity, we don't feel it either—we feel moon-weightless.

Since we are being pulled toward the moon by the moon's gravity, you might wonder why we don't crash into the moon. That's because we're traveling sideways so fast that we perpetually miss the moon and circle it once every 27.3 days. Similarly, the moon perpetually misses the earth and circles it, too.

The only significant effect of the moon's gravity is to create the tide. The earth's oceans are so large that they're sensitive to variations in the moon's gravity. The moon's gravity decreases with distance from the moon, so that the oceans on the near side of the earth are pulled harder than the oceans on the far side of the earth. The result is two bulges in the oceans—one on the near side of the earth and one on the far side of the earth. These bulges create the familiar high and low tides that we observe at the seashore.


618. Hydrogen atoms can form a single bond to each other, oxygen atoms can form a double bond to each other, and nitrogen atoms can form a triple bond to each other. Is there any element that can form a quadruple bond? — KC, Mendenhall, MS
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The bonds that you are referring to are call "covalent bonds," in which two atoms share a pair of electrons in order to lower their total energy. When two electrons are shared in this manner, the electrons are able to spread out over two atoms rather than one. This broadening of their territories lowers their kinetic energies because of quantum mechanical effects. The electrons also spend large portions of their times between the atoms, where they lower the electrostatic potential energies of the two atoms. Lowering the total energy of the two atoms binds them together.

The number of covalent bonds that form between two atoms depends on the number of electrons in those atoms. Hydrogen atoms have only one electron each and can form only one covalent bond. Oxygen atoms have two electrons each that they can share and form two covalent bonds. Nitrogen atoms have three electrons to share and form three covalent bonds. And carbon atoms have four electrons to share, so you might expect them to form four covalent bonds. But there's a hitch...

In the first covalent bond that forms between two atoms, the pair of electrons positions itself directly in between the atoms. This arrangement is most effective for lowering the energy of the system and binding the two atoms together. Chemists call this arrangement a "sigma bond." In the second covalent bond, the two electrons position themselves on both sides of the sigma bond. If you picture the atoms as two people facing one another and holding hands, the electrons are located along the arms of the two people. This arrangement is reasonably effective for lowering the energy of the system and is called a "pi bond." The third covalent bond is also a pi bond, but it forms 90° from the first pi bond, as though the two people are now touching their heads and their feet together along with their hands. With a sigma bond and the two pi bonds between the atoms, there is no room for additional electrons. The fourth covalent bond that two carbon atoms would like to form with one another simply can't form. While two carbon atoms will bind together with a triple bond, each atom will have one remaining electron that is still seeking a partner. The carbon dimer molecule is a highly reactive double radical that will bind to just about anything it encounters.


651. What would happen if the two magnetic poles of the earth were to be reversed? Would it affect climate and weather? Has this ever happened before? — HP, Birmingham, AL
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The earth's magnetic poles have reversed before, many times. A record of the earth's magnetic field is made whenever a magnetic mineral is cooled through a magnetic transition temperature called the Curie point (named after Pierre Curie, the husband of Marie Curie, who first identified it). Volcanic lava often includes such magnetic minerals and as the lava cools, it records a snapshot of the earth's current magnetization. By examine ancient lava flows, scientists have pieced together a detailed record of the earth's magnetization and have found that the earth's magnetic poles have drifted about and reversed many times, typically every few hundred thousand years or so.

I can't think of any mechanism whereby these reversals would seriously affect climate or weather. However, these reversals would affect some migratory animals that use the earth's magnetic field to navigate. In principle, these animals might migrate the wrong direction and die out. However, there are always a few of each species that are born with their magnetic compasses reversed. While these backward animals might not survive during normal times, they would prosper during a reversal and would help to perpetuate their species. Moreover, experiments have shown that individual animals can adapt to the magnetic reversals as well.


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