Category: other

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

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.

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

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.

Who invented the telephone dial or rotary portion of the telephone? — B, R, B, D, and S at Northlake Elementary, Richardson, TX

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.

Can a compound have triple bonds? If so, please give an example. — BA, IL

Yes, some compounds contain triple bonds. Acetylene is the simplest such molecule, with two carbon atoms connected by a triple bond. Each carbon atom has one hydrogen atom attached to it, so the entire molecule is a four-atom chain: hydrogen-carbon-carbon-hydrogen. The triple bond between carbon atoms is extremely strong—the atoms are sharing 6 electrons between them.

What is red mercury, where does it come from, and where is it used?

Mercuric sulfide, a red mineral known as cinnabar, is the world’s principal source of mercury and has also been used frequently as a vermilion paint pigment. It has been mined in Almaden, Spain for several thousand years and occurs in a number of other countries, including the United States.

I’ve seen tops that rest with their large parts down but that flip up onto their handles when you spin them. What is the reason that they have a different equilibrium when they are spinning versus when they are not? — CH, Renton, WA

While I’m not an expert on these “tipple tops,” I believe that I understand how they work. These tops have large round heads and look like wooden mushrooms. When you hold the handle (the mushroom’s stem) and spin it with its head down, it quickly flips over so that it spins on its handle. The flipping is caused by a torque that friction exerts on the top’s round head as the tops surface slides across the table. If the top were perfectly vertical as it spun on its head, friction between the top and the table would exert a torque (a twist) on the top that would simply slow the top’s rotation. But when the top isn’t perfectly vertical, the torque that friction exerts on it does more than slow its rotation. This torque also causes the top to precess (change its axis of rotation) in such a way that the top’s handle gradually becomes lower and the top’s head gradually becomes higher. Eventually, the top’s axis of rotation inverts completely so that it begins to rotate on its handle. Once that happens, the precession stops because the handle is too narrow for anything but the slowing effects. Only when the top stops spinning does it shift from this dynamically stable arrangement (handle down) to its statically stable arrangement (head down).

How does a UPC scanner work?

UPC labels are the bar codes placed on consumer goods to identify them as they pass over a glass window containing a UPC scanner. Although UPC labels were first conceived by Norman Joseph Woodland in the late 1940’s, the scheme to read those codes required a very bright and narrow beam of light that could be scanned rapidly across the bars in order to measure their widths. Conventional light sources barely worked and the idea didn’t catch on until lasers became available. A modern UPC scanner begins with a laser that emits a tightly collimated beam of light. Early scanners used helium-neon lasers, but new scanners use cheaper and more reliable solid-state or diode lasers. In a typical scanner, the red beam from a laser is directed toward a spinning object—either a carefully faceted and mirrored disk or a flat disk containing a carefully designed hologram. Laser light that reflects from the spinning object emerges from the glass window above the scanner and sweeps rapidly through the space like a tiny searchlight. When this light beam encounters a UPC label, each dark bars absorbs the beam while each light bar reflects it. Thus as the beam scans across the UPC label, the amount of light the product reflects fluctuates up and down in a characteristic manner. When a photodetector in the UPC scanner detects such a fluctuating reflected light signal, it determines that the laser beam is hitting a UPC label. A computer studies the sequence of the light and dark bars to determine exactly what UPC label is being hit and identifies the product to the store’s computers.

Are divining rods and their abilities to locate ground water fact or myth?

I’m afraid that I think they’re myth. Despite extensive searches, physicists have found only four forces in nature: gravity, the electromagnetic force, the strong force, and the weak force. Of these, only gravity and the electromagnetic force are noticeable outside of atoms. Since ground water has no electric charge, it can’t affect a divining rod through the electromagnetic force. That leaves only gravity as a possibility and the gravity between modest sized objects such as a stick and a pool of water is so incredibly weak that I can’t imagine anyone detecting it with their hands. Having eliminated all the possible external forces that would bend a stick downward when it’s near water, it’s clear that this bending is done by the hands of the person holding it. Perhaps a good dowser can see features in the environment that prompts the dowser, consciously or unconsciously, to believe that water is nearby. In short, I think that there are people who are good at identifying signs that indicate ground water is present and who can find that water. The divining rod itself is unimportant.

What are the two substances in a Lava Lamp, and why do they react the way they do?

I’m afraid that I’m unable to determine exactly what substances the lamp contains. However, I believe that one of them is water and the other is a high-density wax. When the lamp is cold, the wax is a crystal solid with a density slightly higher than that of water. Because the buoyant force this wax experiences from the water is less than its weight, the wax sinks to the bottom of the lamp. But when the lamp is on, the bottom of its container heats up and the wax begins to melt. Like most materials, wax’s liquid phase is substantially less dense than its solid phase. As it melts, the wax expands so much that its density drops below the density of water and it floats upward to the cool top of the container. Once it reaches the top, the wax begins to solidify. As it solidifies, the wax contracts so much that its density rises above the density of water and it sinks downward to the bottom of the container. Thus when the lamp is in full operation, the rising bubbles of wax are liquid and the descending bubbles of wax are solid. Dyes are added to the two materials to make them more visible—the water is colored by a water-soluble dye (perhaps food coloring) while the wax is colored by an oil-soluble dye (like those used in permanent markers).

How does a mass spectrometer work and why must it be evacuated before being used?

A mass spectrometer is a device that measures the masses of the atoms or molecules in a sample. There are many different types of mass spectrometers but they all work on roughly the same principle: they give each atom or molecule a single electric charge and look at how easy or hard it is to accelerate that atom or molecule by pushing on it with electric or magnetic fields. The more mass the atom or molecule has, the more slowly it will accelerate in response to a particular force. Some mass spectrometers use an electric field to push the atoms or molecules forward until they all have the same amount of kinetic energy and the more massive particles end up traveling more slowly than the less massive particles. Their masses can then be determined by timing how long it takes them to travel a certain distance or by sending them through a magnetic field that bends their flight paths. Because the force that a magnetic field exerts on a moving particle increases with that particle’s speed, the paths of slow moving massive particles bend less than those of fast moving less massive particles. Since all of this mass analysis occurs while the particles are traveling through space, it’s important that they not collide with any gas particles inside the mass spectrometer. That’s why the mass spectrometer must be evacuated before use.