Thermal heating

Chapter Thermal heating

Teach Yourself Electricity and Electronics Third Edition Book
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Teach Yourself Electricity and Electronics Third Edition Book

  • be employed, so that electrostatic forces can operate against tension springs or mag-nets, and in this way, electrostatic meters can be made.An electrostatic device has the ability to measure alternating electric charges aswell as steady charges. This gives electrostatic meters an advantage over electromag-netic meters (galvanometers). If you connect ac to the coil of the galvanometer devicein Fig. 3-1 or Fig. 3-2, the compass needle might vibrate, but will not give a clear de-flection. This is because current in one direction pulls the meter needle one way, andcurrent in the other direction will deflect the needle the opposite way. But if an alter-nating electric field is connected to an electrostatic meter, the plates will repel whetherthe charge is positive or negative. The deflection will be steady, therefore, with ac aswell as with dc.Most electroscopes aren’t sensitive enough to show much deflection with ordinary117-V utility voltage. Don’t try connecting 117 V to an electroscope anyway; it might notdeflect the foil leaves, but it can certainly present a danger to your body if you bring itout to points where you can readily come into physical contact with it.An electrostatic meter has another property that is sometimes an advantage inelectrical or electronic work. This is the fact that the device does not draw any current,except a tiny amount at first, needed to put a charge on the plates. Sometimes, an en-gineer or experimenter doesn’t want the measuring device to draw current, becausethis affects the behavior of the circuit under test. Galvanometers, by contrast, alwaysneed at least a little bit of current in order to operate. You can observe this effect bycharging up a laboratory electroscope, say with a glass rod that has been rubbed againsta cloth. When the rod is pulled away from the electroscope, the foil leaves will remainstanding apart. The charge just sits there. If the electroscope drew any current, theleaves would immediately fall back together again, just as the galvanometer compassneedle returns to magnetic north the instant you take the wire from the battery.Thermal heatingAnother phenomenon, sometimes useful in the measurement of electric currents, is thefact that whenever current flows through a conductor having any resistance, that con-ductor is heated. All conductors have some resistance; none are perfect. The extent ofthis heating is proportional to the amount of current being carried by the wire.By choosing just the right metal or alloy, and by making the wire a certain lengthand diameter, and by employing a sensitive thermometer, and by putting the entire as-sembly inside a thermally insulating package, a hot-wire meter can be made. Thehot-wire meter can measure ac as well as dc, because the current-heating phenomenondoes not depend on the direction of current flow.A variation of the hot-wire principle can be used by placing two different metalsinto contact with each other. If the right metals are chosen, the junction will heat upwhen a current flows through it. This is called the thermocouple principle. As with thehot-wire meter, a thermometer can be used to measure the extent of the heating.But there is also another effect. A thermocouple, when it gets warm, generates a di-rect current. This current can be measured by a more conventional, dc type meter. Thismethod is useful when it is necessary to have a faster meter response time. Thehot-wire and thermocouple effects are used occasionally to measure current at radiofrequencies, in the range of hundreds of kilohertz up to tens of gigahertz.Thermal heating47