GEISSLER TUBE of SIMPLE CONSTRUCTION LOUIS AUERBACH
AND
HERMAN KAMMIN
Grover Cleveland High Schwl, New York City
A
LTHOUGH the Geissler tube is a valuable tool for use in demonstration on physics and chemistry lecture tables, its construction is usually thought to necessitate the use of high-vacuum pumps. A simple Geissler tube, which may be constructed by anyone with a slight knowledge of glass blowing, is the subject of this paper. It is the more interesting in that it is of rather wide application, as it can be used to demonstrate the principle of the barometer, the simple gas discharge tube in physics, and the method of producing a glow discharge in gases in general science and chemistry. The method herein described is the oldest method ever used to produce evacuated chambers and was used by Torricelli and his associates in the seventeenth century in their researches upon the properties of evacuated bodies.' They did not, however, seal electrodes through glass but produced gas discharges by producing electrostatic charges on the walls of the vessel either by rubbing it or by splashing mercury in the evacuated vessel.
This arrangement has the advantage that suction can be applied to the side-arm and the vacuum in the discharge tube thus varied at will before it is finally sealed off. Two major disadvantages of this Geissler tube are that only an air-mercury discharge tube can be produced while, secondly, a barometric vacuum (0.001mm. Hg) is never attained because of the minute bubbles of air which are trapped in the apparatus and reduce the vacuum attained. In most cases, however, a pressure of less than 10 mm. Hg can be reached, this last pressure representing the highest one with which a satisfactory Geissler discharge can be obtained. The fall-tube can be easily sealed into the Geissler tube by drawing down the wide tube, to be used as a Geissler tube, a t one end and sealing the fall-tube to FIGURE 2 the narrowed glass tube. D-Vacuous space This operation is best performed by heating the glass ends to be joined in a CONSTRUCTION brush-flame on the blast lamp, touching the ends of the The method of construction is evident from Figure 1. tubes together, and then going over the joint thorThe apparatus, A B C, is inverted so that the elec- oughly with a hot needle flame. A fully detailed detrode chamber is a t the bottom and then filled with scription of this operation and those to be described immediately following will be found in any text on mercury. A funnel drawn out to a capilglass b l o ~ i n g . ~ J lary tip works very well. The thumb of Briefly, the process of sealing in the electrodes* is as the operator is then firmly placed over A follows. The tube is drawn down to a small aperture the open end of the vertical fall tube, where the electrode is to be sealed. In the event that the apparatus is re-inverted, and the end the point is a t the side of a tube, the spot is heated hot C is plunged under the surface of the with a small flame, a glass rod is touched to it, and a mercury pool before the thumb is rethin capillary is pulled out, using the rod as a handle. leased. The apparatus then should preThe capillary is then broken off close to the main tube. sent the appearance represented in The electrode (platinum or dumet) is slipped into a Figure 1. The mercury should sink far short section of thin capillary tube (previously drawn enough in the tube so that the bulb FIGURE 1 down from ordinary tubes) heated until the glass melts can be sealed off with a hand torch. and adheres perfectly to the electrode. The glassAn alternative method is to fill the A &B-Electrodrs sheathed wire is then inserted into the aperture already bottom of a side-arm test-tube with mol- E-Mercury prepared in the discharge tube, and the tube is melted ten beeswax and while it is still soft to place it over the end of the inverted (and filled) down upon it. If a temporary installation is desired, apparatus so that the end of the tube C is covered the electrodes can be sealed in with De Khotinsky cewith the beeswax, while a t the same time a one-hole ment, which will keep the degree of vacuum desired several days under favorable conditions. rubber stopper previously slipped over the tube C for closes the mouth of the test-tube. Now invert the FRARY. TAYLOR, AND EDWARDS. "Laboratory glass blowing,'' entire device; the mercury column will be capped 2nd- .ed., McGraw-Hill Book Co., Inc., New York City, 1928, el sep. by the beeswax until the apparatus is upright when p. 94,LEVITT, "Pyrex bends bulbs and seals," Corning Glau the weight of mercury will cause it to break loose from Works, Corning, N . Y. * An acceptable electrode can be salvaged from the press of a the beeswax and drop to a barometric column. burnt-out electric lieht bulb or radio tube. Onlv the iron or ' KAYE, G. W. C., "High vacua." 1st ed., Longmans, Green & nickel end must be-sealed in, however, as a copper wire will
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Co., New York City, 1927, p. 10.
amalgamate with the mercury.
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