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Electrode Selection

How to match the proper electrode to a neon tube



In many failed sign installations, I found an improperly selected electrode caused the defect. Also, some neon-school instructors use incorrect phrases, such as, "Use long electrodes for long tubes, short ones for short tubes." Hence, before you remake the tubes and face the same problem again, I will discuss how to select the appropriate electrode and its physical properties.

Size considerations

As with many life situations, one of the first things you’ll notice about neon electrodes is that they exist in different sizes (Fig.1). Being rational beings, we look for a reason for their variety, and the answer can be found in history. In 1911, George Claude’s invention of the neon tube started the development of the electric sign and illumination. In his patent, he stated that the electrode’s surface size must be related to the operating current to achieve a useful lifetime.

This can be easily understood by regarding the electrode as the electrical connection to the gas. The connection occurs at the point the gas touches the electrode. Every electrical connection with a given cross section can be loaded only to a certain extent without overheating. The critical cross section where the highest resistance occurs is the surface of the electrode. A stronger operating current requires a larger electrode surface so that it will not overheat or be destroyed.

As said, a surface area can carry only a specific amount of operating current, technically stated as the "current density" on the surface, measured in milliamps (mA) per square millimeter. Claude’s electrodes were limited to 6.5mA per square centimeter, whereas today 12 to 15 mA per square centimeter are achieved.

Long and thin or short and thick

In an operating neon electrode, the gas discharge emerges only from inside of the metallic part of the electrode. Immediately a question arises: Why is there no glow on the outer surface? Because electricity behaves like many human beings — it travels the path of least resistance.

In my article, "Neon Electrodes – Past and Present," I explained that the phenomenon of the hollow-cathode effect causes a higher resistance to electrical current flow from the outside, compared to the inside. It’s easier for electricity to emerge from the inside of the hollow electrode into the gas discharge. Therefore, only the inner surface counts for the "active" surface area.

Fig. 2 shows two electrodes with the same surface area. Which electrode should be chosen for the given surface area and, respectively, operating current rating? With long and thin tubular shell shapes, all electrons and ions must pass through a small orifice, whereas short and wide shells, which don’t provide the full hollow-cathode effect, tend to sputter more easily. Thus, selecting extreme dimensions may create problems (mainly for processing than for operating), but in general it makes no difference if you use a long-and-thin or a short-and-wide shell electrode for the same current rating. Also, the shape exerts a negligible influence if physical principles are not neglected (Fig. 3).

Electrode activation

Claude’s patent claimed that a minimum electrode surface per current is necessary. As his business grew quickly, competitors tried to bypass his patent by making a reliable electrode with a smaller surface. In the late 1920s Kresge and Cortese found that coating the surface with earth alkali oxides — a principle found by Wehnelt in 1907 for X-ray and cathode-ray tubes — produced reliable operating results in neon tubes.

Nearly all neon electrodes used currently contain this type of coating on the inside of the shell. The coating, called "activation," activates electrons so that they are more readily liberated from the metal into the vacuum than from uncoated surfaces. As Fig. 3 demonstrates, the size necessary for a given current rating is remarkably reduced using activation.

Bombardability, another constraint

Before you operate a neon tube at a given current with a selected electrode, the tube must be manufactured, too. In fabricating a neon tube, the bombarding process is the most important factor in creating a quality product, beside the glassworker’s skill.

The activation mass inside the electrode is not stable under atmospheric conditions, so it is introduced as a stable chemical compound which must be converted to form the activation mass during bombarding. This process requires a minimum temperature for the metallic shell to reach to obtain this conversion. On the other hand, the glass needs to be heated to about 375



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