122. External Electrodes Discharge through Higher Vacuum than Internal.—A vacuum that was so high that no discharge took place with internal electrodes was made luminous by the use of electrodes on the outside of the glass bulb. Then he made the vacuum so high that even with a 12-inch spark from Leyden jars, no discharge took place with external electrodes, and the tube was dark, this part of the experiment indicating another limit at which an extremely high vacuum is not a conductor and appearing to overthrow, as Edison intimated, Edlund’s theory that a vacuum is a perfect conductor. [§ 25].
123. Deposit on Glass from Aluminum Electrode.—It has always been common to employ aluminum for electrodes in vacuum tubes, on the ground that no deposit took place, and therefore no blackening, nor whitening of the glass wall. [§ 40]. Edison observed also that no blackening was visible, but stated that his glass blower, Mr. Dally, upon breaking the bulb and submitting the interior surface of the glass to an oxydizing process, the oxide of aluminum was so thick as to be opaque to light. With magnesium, also, a mirror was produced, of a lavender color, by transmitted light. In the case of aluminum, he was able to obtain a visible spot at the phosphorescent portion, but only after a great many hours of use. See cut from a photograph of a discharge tube used for several months by Prof. Dayton C. Miller, and having a heavy aluminum deposit opposite the aluminum cathode. With the increase of the deposit, the power of the X-rays diminished, but, he thought, not on account of the absorption, but because, “through lack of elasticity at the surface.”
Discharge Tube, [§ 123].
124. Fluorescent Lamp. In an English patent of ’82, granted to Rankin Kennedy, there is described a vacuum bulb in which the electrodes are covered with fluorescent or phosphorescent substances, intended for the purpose of obtaining greater candle power by impact of cathode rays upon anode of platinum, covered with alumina or magnesia. Edison coated the inner wall of the discharge tube, for generating X-rays, with calcic tungstate in the crystalline form. The luminosity, when measured, amounted to about 2-1/2 C. P. As to the efficiency, he stated that this was accomplished “with an extremely small amount of energy.” Such a coating was found to weaken the X-rays radiated therefrom, which, of course, was natural, because they had been converted into phosphorescent light. The spectrum showed strongly at the red line, thereby suggesting the reason why the light was of a pleasant character.
124a. Piltchikoff’s Experiment. Greater emission of X-rays by a tube containing an easily fluorescent substance. Comptes Rendus, Feb., 24, ’96. From trans. by Mr. Louis M. Pignolet. As the X-rays emanate from the fluorescent spots on the glass of the discharge tube, he reasoned that more powerful effects would be obtained by replacing the glass by a more fluorescent material. He therefore tried a Puluj tube and found that it shortened the time necessary for taking a photograph in a “singular” degree. Experiments of others have certainly shown that as phosphorescence decreases with increase of vacuum, the X-rays increase to a certain maximum, [§ 105]. Let it be noticed however, that this does not prove that with the same vacuum, an increase of phosphorescence by a superior phosphorescent material of equal thickness would not increase the power of the X-rays. The best way to determine such points, is to go to extremes. Edison applied so much easily phosphorescent material (calcic tungstate) to the inside of the discharge tube, that much light was radiated, but only feeble X-rays. On the other hand, without any of the tungstate, the rays were strong, [§ 124]. Experiments generally tend to prove that it depends upon the chemical nature of the material rather than its phosphorescing power, in other words upon the permeability. [§ 119], near end.
125. Electrodes of Silicon Carbide. (Carborundum.) Edison called attention to Tesla’s discovery that this substance is a good conductor for high tension currents. Its advantages for electrodes in the discharge tube are its high conductivity, no absorbed nor released gas bubbles, and its infusibility and non-blackening power of glass even when the voltage was increased to a point where the glass melted.
Edison (at right) and T. Commerford Martin using the Sciascope. [§ 97], p. [84].
Cut also shows Sprengel vacuum-pump. Discharge-tube is in the box.
126. Chemical Decomposition of the Glass Bulb. During the generation of the X-rays the sodium line of the spectrum appeared in the spectroscope, thereby indicating decomposition of the glass. With combustion tubes the glass gave the weakest soda line, while lime soda glass gave the strongest, and was most permeable to the X-rays. “The continuous decomposition of the glass makes it almost impossible to maintain a vacuum except when connected to the pump and even then the effect of the current is greater in producing gas than the capacity of the pump to exhaust, but the ray is very powerful.” It is supposed that for this reason, as well as for others easily apparent that Edison as well as other experimenters have always carried on their investigations with the discharge tube permanently connected to the pump. The next best thing is to let the tube contain a stick of caustic potash for maintaining an exceedingly high vacuum. By gradually heating this, the desired degree of vacuum can be obtained. [§ 54].