Page 67 - Teach Yourself Electricity and Electronics

Thermal heating 47

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 as
well as steady charges. This gives electrostatic meters an advantage over electromag-
netic meters (galvanometers). If you connect ac to the coil of the galvanometer device
in 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, and
current 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 whether
the charge is positive or negative. The deflection will be steady, therefore, with ac as
well as with dc.
Most electroscopes aren’t sensitive enough to show much deflection with ordinary
117-V utility voltage. Don’t try connecting 117 V to an electroscope anyway; it might not
deflect the foil leaves, but it can certainly present a danger to your body if you bring it
out to points where you can readily come into physical contact with it.
An electrostatic meter has another property that is sometimes an advantage in
electrical 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, because
this affects the behavior of the circuit under test. Galvanometers, by contrast, always
need at least a little bit of current in order to operate. You can observe this effect by
charging up a laboratory electroscope, say with a glass rod that has been rubbed against
a cloth. When the rod is pulled away from the electroscope, the foil leaves will remain
standing apart. The charge just sits there. If the electroscope drew any current, the
leaves would immediately fall back together again, just as the galvanometer compass
needle returns to magnetic north the instant you take the wire from the battery.

Thermal heating

Another phenomenon, sometimes useful in the measurement of electric currents, is the
fact 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 of
this 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 length
and 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. The
hot-wire meter can measure ac as well as dc, because the current-heating phenomenon
does not depend on the direction of current flow.
A variation of the hot-wire principle can be used by placing two different metals
into contact with each other. If the right metals are chosen, the junction will heat up
when a current flows through it. This is called the thermocouple principle. As with the
hot-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. This
method is useful when it is necessary to have a faster meter response time. The
hot-wire and thermocouple effects are used occasionally to measure current at radio
frequencies, in the range of hundreds of kilohertz up to tens of gigahertz.