Category: other

What is DTMF and how can I measure the pulses on a rotary phone?

DTMF is short for “Dual Tone MultiFrequency” and refers to the pair of tones that a telephone uses to send dialing information to the telephone switching system. Each time you press one of the buttons on the telephone, it emits two tones simultaneously. A decoder at the other end recognizes these two tones and determines what button you pushed. One tone is associated with the button’s row and one tone with the button’s column. Since there are four rows of buttons, there are 4 possible row tones and since there are three columns of buttons, there are 3 possible column tones. A fourth column of buttons, A through D, and a fourth column tone are part of the specifications for DTMF but do not appear in normal telephones. Naturally, all 8 tones are different and the web has countless pages that discuss these tones (touch here for an example)

As for measuring the pulses on a rotary phone, you can do this if you can study the telephone’s electric impedance (or resistance). As the dial switch turns, it briefly hangs up the telephone repeatedly. The number of hangups is equal to the number you are dialing (although dialing “0” causes it to hang up 10 times). You can actually dial by hanging up the telephone rhythmically and rapidly several times. If you click the hang-up button 5 times rapidly, you will dial a “5”. To detect that this hanging up is happening electronically, measure the telephone’s impedance—the impedance rises dramatically during each hang-up. If there is a constant current passing through the telephone, the voltage across its two wires will rise. If there is a constant voltage reaching the telephone, the current passing through it will drop. The telephone company detects this repeated change in impedance and determines what number you dialed.

When astronomers study sunspots they occasionally notice that there only seems to be one magnetic pole. But I thought that monopoles didn’t exist that we know of. What’s going on?

While a sunspot may have only one magnetic pole associated with it, there is sure to be an equal but opposite pole somewhere else in the sun. Probably it’s located deep inside the sun or somewhere else on the sun’s surface. Like one end of a long bar magnet, the sunspot looks like a single pole, but it’s really connected to an equal but opposite pole.

How do black holes work?

As you assemble more and more mass together in a small volume, the gravity there becomes stronger and stronger. At first, it becomes more and more difficult to throw a ball upward hard enough to make it sail away from the mass into space. Eventually, you need a cannon to get the ball to leave. And by the time you get enough mass together, the gravity becomes so strong that light itself begins to have trouble escaping. Light falls in gravity, just like anything else. But it travels so fast that you barely notice it falling. However when the gravity becomes strong enough, light falls enough to cause some weird effects. A black hole forms when the gravity is so strong the even light is unable to escape from the mass.