Radio Broadcast (May 1927-Apr 1928)

JANUARY, 1928 RADIO ENLISTS THE HELIUM ATOM 197 sprayed alternately to the anode elements as these become positive in turn under the inductive action of the a. c. current in the primary .powersupply windings. The filter chokes and tanks prevent a voltage and current fluctuation in the line during the intervals between the spraying or emission surges through the rectifier tube. So deeply ingrown is the current-flow conception that in a recent issue of a well-known radio magazine there was a cut of a helium-gas tube with the hat-shaped cathode marked "anode" and the two anodes marked "cathodes." The accompanying text used the same terminology, which was wrong even in the light of the old theory. It should be clear that the positive side of a B-device filter is of negative potential compared to the ends of the secondary winding of the power transformer to which the rectifier-tube anodes are connected. The large cathode of a helium rectifier is equivalent to the incandescent filament of the filament type tube. WHY A TUBE DETERIORATES CINCE the helium-gas rectifier tube contains ^ no heated filament emitter to burn out or become lifeless through deterioration, many usersof thedevice feel that it ought to last almost forever — for years and years at any rate, and wonder why it sometimes has a short life. As a matter of fact, the developers of the tube themselves thought at first that it would have a life of 10,000 hours or more of continuous use, but soon found that such was not the case. The principal thing that brings the life of a heliumgas rectifier tube to an end is the fact that the helium gas in the bulb disappears. Helium gas is inert, it will not combine with anything, so far as we know at present; it is genuinely strange, therefore, that it should disappear from a hermetically-sealed bulb. It seems that the ionized gas particles pound the cathode with such force that some of them are driven deep into the metal and stay there — become occluded or imprisoned. After a certain length of time so many gas atoms are bound in the metal that the tube becomes very hard, the vacuum rises, and the bombardment of the cathode becomes meager, owing to the reduced number of molecules of gas to do the battering. The current output of the tube then falls off to such a point that it must be discarded. The life of many a good helium-gas tube has been quickly brought to an end through the breaking down of condensers in filter circuits. Cheap inferior condensers in both home-made and factory-built power devices usually go to pieces after a few weeks or months of use, with the result that the rectifier tube is placed in a dead short-circuit. The heavy current flow quickly burns off the tips of the anodes in the tube. The helium gas itself cannot be injured by any current. Helium gas will carry currents so great that they will instantly explode copper conductors of the same cross-sectional area; but under such currents the gas particles quickly drive themselves deep into the negative electrodes and are as good as lost. Some of the cheaper helium-gas tubes now on the market may be short-lived through the presence of impurities in the helium, which would destroy the electrodes. Extremely pure helium must be used. This gas is purified by passing it through copper tubes filled with cocoanut charcoal and maintained at the temperature of liquid air — more than 2500 below zero, Fahrenheit; then to a steel reservoir; then to a second battery of tubes of charcoal surrounded with liquid air to remove oxygen or nitrogen which might come away from the walls of the reservoir. The helium is admitted until the charcoal is partly loaded with it; next it is pumped off with vacuum pumps and the impurities remain in the charcoal, which is itself purified and used over again. It is interesting to consider that absolute purification of anything, liquid, gaseous, or solid, is almost impossible. Imagine for a moment that you could mark molecules in some such way that you could identify them when you saw them again. Assume that you took a glass of water with the molecules of water all thus marked and stirred it into the waters of the oceans of this earth, that you waited a couple of million years for thorough mixture, and that you then walked up to the nearest hydrant in your vicinity and casually drew a glass of water, you would find about 2000 of your marked molecules in it! Of course, we may be a few molecules out on this estimate, but that is roughly the correct mathematical number, because there are 2000 times as many molecules in a glass of water than there are glasses of water on the earth. Again, molecules of air, if admitted into a highly-evacuated 25-watt electric light bulb through a hole so small that they had to flow in single file, would take about 100,000,000 years to fill it to atmospheric pressure. A little reflection will show that "purity," like everything else, is probably only a relative condition. "Some of 'em is more pure, and some of 'em is less pure, but none of 'em is all pure," said the sour cynic, and while he wasn't speaking of gases, and was not a scientist for that matter, he was uttering profound truth. THE TIN "HAT" \ A /HAT is the queer tin "hat" for in the * * modern gaseous rectifier tube? Nobody outside of the research laboratories seems to know. Even some of the "bootleggers" making these tubes don't know. In the early experimental forms of the heliumgas tube, great trouble was met with owing to the disruption of the cathode element under the hammering of the ionized gas atoms. The earlier tubes had disk shaped cathodes, and the gas atoms pulled out electrons with such violence that tiny pieces of solid metal were often ripped loose from the electrode. These metal bits were thrown against the glass walls of the tube, blackening it,' and resulting in the speedy destruction of the cathode. For this reason the peculiar tin-hat form of cathode was evolved because, with this arrangement, the bombardment of the cathode and emission of electrons is entirely internal. The action all takes place inside of the electrode. If a bit of metal is torn from the cathode at any point, it is hurled across the inner chamber and thrown back onto the element somewhere else. There is no loss of metal because the cathode is continually built up as fast as it is torn down. It seems that, if we, as human beings, could be temporarily reduced to the size of, say, an atom, together with a corresponding ability to see small objects, and were then placed upon the cathode of an operating helium-gas rectifier tube, we would have an impression of standing among ranges of heaving mountains of metal in a state of furious convulsion and uproar, bombarded with enormous meteors of helium and full of volcanic upheavals and earthquake-like shocks, while electrons would arise like clouds of steam on all sides, or spurt out like fiery sparks. A VIEW OF THE LABORATORY IN WHICH THE HELIUM-GAS RECTIFIER TUBE WAS PERFECTED