Neon bulb

Louis A. Bloomfield s entertaining book How Things Work The Physics of Everyday Life (second edition), Wiley, New York, 2001, discusses neon bulbs and fluorescent strip lighting, see pp. 395-399. For a more scientific look at fluorescent dyes, try Chapter 3 of Peter Bamfield s Chromic Phenomena (above), especially pp. 182-184. 

Temperature should be strictly controlled in the microwave oven with a temperature probe that has a feedback mechanism to regulate the energy output of the microwave oven and thus maintains the optimal temperature. Alternatively, temperature can be controlled by placing a water load in the chamber of the microwave oven, which absorbs extra energy and provides humidity, slowing the evaporation of reagents. In addition, hot spots in the chamber should be avoided by using the neon bulb display method (Chapter 5). 

Figure 11. Fidl try cock-neon bulb device...

Figure 10.1. Spectra of neon bulb emission, expressed as Raman shift (in air) relative to 514.532 nm. Obtained with Spex 1402 double monochromator, RCA 31034 PMT. Indicated values were calculated from Reference 2.

Now attach both wires of the neon tester to both terminals of the 9 volt battery, either with or without clip leads. (Of course, this is not a dangerous part of the experiment, since 9 volts is very unlikely to hurt anyone with reasonably dry skin, but it still makes good sense to avoid any two-handed contact of metal parts having voltage on them.) The neon bulb does not light up, even faintly. 

Figure 1.3 Using the inductive kick to light a neon bulb.

When initial contact is made by this makeshift switch, nothing visible happens. But when the contact is broken, the neon bulb lights up for an instant, similar to the spark experiment described previously. Therefore the inductor has generated more than 90 volts, from only a 9 volt battery. At this... 

This neon bulb s characteristic curve is sometimes referred to as being "non-ohmic." That term is not a very clear description, because at any given point on the curve. Ohm s law still applies, and R = V/I. A more descriptive statement would be that the resistance is not constant. At a high enough voltage, the neon gas begins to ionize more and more (an "avalanche" effect), and the resistance goes down. 

Gas filled tubes such as some types of neon bulbs (and also some silicon diode devices) have characteristic curves that are "flatter" and more horizontal along the top than the one shown here. Examples of these will be studied later in the course. Those devices can be used to ensure that the voltage remains constant, over a wide range of currents. 

If the circuit of Fig. 10.2 on the previous page is assembled with the 9 volts going into the primary (two heavily insulated black wires), it will still be used in the "step-down" mode, and the neon bulb will not light when hooked up to the secondary (lightly insulated yellow wires), because it requires much more than 9 volts, and instead it will probably be getting less. (Actually, it could be lit if the battery was connected or disconnected fast enough to make up for the unfavorable ratio of turns, but probably neither operation can be done fast enough here.)... 

The experimenter should construct the circuit of Fig. 10,5 on the next page. At first, the wires shown as dashed lines are not connected. The experiment is done as on page 7 of the first chapter, where the "switch" is opened suddenly, and there is a bright flash in the neon bulb. Then the short circuiting clip lead (dashed line in the diagram) is attached to the transformer s 120 volt coil (heavily insulated black wires or soldered-on power cord). Now when the switch is opened, there is no neon flash. What is happening is that current is induced in the "shorted" coil, as the battery current suddenly decreases. This new current has a magnetic field... 

This type of simple circuit (without the neon bulb or capacitor) is commonly used to operate doorbells, with a bell clapper attached to the armature, which hits a metal bell. It was also used for the older model telephone "ringers," before modern transistorized oscillators became available. Without the clapper and bell, it is still used a lot for various "buzzer" applications. (More modem buzzer designs use the piezoelectric diodes discussed in Chapters 14 and 15.)... 

A neon bulb is a "gas tube diode," in which an electron avalanche takes place in a partial vacuum containing a small amount of the element neon. There is no difference between the two electrodes, so the same V-I curve is followed in both... 

Another type of avalanche diode is the "diac." This also has a characteristic curve that is the same in both directions, because it is a symmetrical design with a PNP structure. It can be thought of as two PN diodes, back to back. Each one is designed to be able to avalanche at about 1 ma of current without being damaged. Breakdown can be as low as about 6 volts, and this goes lower after the avalanche has started, even more so than in the neon bulb. We will make use of this device in Chapter 21. As an optional experiment, it can be inserted into the curve tracer to give a curve like that in Fig. 14.4. 

A confusing term, since true negative resistance does not exist. (If it did, energy could be generated from nothing.) A better description of the phenomenon is "negative differential resistance," in which a resistance decreases when the current increases. Examples are neon bulbs and PNPN diodes, just after the current has increased sufficiently to "turn them on."... 

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