Table of Contents
Tungsten to Helium
Gerald L. Wendt and Clarence E. Irion (University of Chicago) reported their "Experimental Attempts to Decompose Tungsten at High Temperatures" to a meeting of the American Chemical Society in Illinois in April 1922. (4, 5)
Wendt and Irion claimed to have completely disintegrated tungsten wire into helium by means of a high-voltage discharge in glass bulbs. In the mean of 21 experiments, 1.01 cc of helium was obtained from a wire length of 39.62 mm with a weight of 0.713 mg, exploded with 29.6 kilovolts. The procedure consisted of charging a condensor to 100 Kv and discharging it at high speed through an extremely fine wire. The resulting explosion generated a pressure of about 1,000 lb/in2 and a temperature over 50,000o F. The method introduced as much as a coulomb of electricity into the tungsten wire within 1/300,000th of a second. The accompanying flash of light was about 200 times as bright as sunlight, and lasted less than 1/100,000 of a second. No smoke or other residue was ever found after the explosions.
Wendt and Irion described the electrical circuit and bulb (Fig. 4.1, 4.2) as follows:
"The primary circuit of the transformer, T, operates on a 220-volt alternating current power line through an inductive resistance, E. In order to prevent a destructive back-pulse into the power line should the charged condenser accidentally be discharged through the secondary circuit of the transformer, 2 condensers of 1 microfarad capacity each are bridged across the primary circuit with a ground connection, as shown at A. The primary circuit was heavy enough to carry 40 amperes during the brief period necessary to charge the large condenser; the secondary circuit furnished 100,000 volts though ordinarily only some 30,000 were used. The secondary circuit was connected to the two sides of the large condenser, C, one side leading through the hot cathode 'kenotron' rectifier, R, which was especially designed for heavy service and a large factor of safety. Its cathode filament was heated by the battery of dry cells, B. The discharge circuit led from the two sides of the condenser and contained only the spark gap, S, and the wire to be exploded, H. This discharge circuit was made as short and compact as possible, of heavy copper strip, in order to reduce resistance and inductance to a minimum and thus allow a rapid and non-oscillating discharge through the wire in the minimum time, thus concentrating the energy input and giving the maximum temperature in the material to the wire. To give maximum capacity and hold maximum voltage the condenser was built of 100 glass plates 60 by 75 cm covered with heavy tin foil and cast into solid paraffin with a gap of 5 mm between plates. The condenser showed brushing at the edges of the plates at 30,000 volts but held 45,000 volts without puncturing. The capacity was about 0.1 microfarad. The spark gap consisted of two 2 cm brass spheres, their separation adjustable to the maximum voltage of the condenser. Its use is important since it is the only means for protecting the condenser from excessive charge by the transformer, and for insuring a complete and sharp discharge at the proper moment.
"Tungsten was chosen for the material of the wire to be exploded chiefly because its high atomic weight made its decomposition probable on the hypothesis adopted, and also because it is hard enough to allow convenient manipulation and support even in excessively thin wires. The wires were 0.035 mm in diameter, about 4 cm long and weighed 0.5 to 0.7 mg. They had sufficient strength to be sprung into place between the larger electrodes shown in Fig. [4.2] without welding or clamping.
"The construction of the explosion bulb is shown in Fig. [4.2]. It has a volume of about 300 cc, and was constructed of heavy Pyrex glass without strain and in good spherical form, for it was required to withstand momentarily a tremendous outward pressure. Thick bulbs invariably broke during the explosion because of insufficient elasticity. Thin bulbs may be used is the bulb is immersed in a vessel of water, which gives sufficient support together with elasticity. The large side-tube is the neck at which the bulb was sealed from the pump system after evacuation, and through which the wire was sprung into place between the electrodes by means of pincers. The smaller side tube contained a third sealed-in electrode, and served for the spectroscopic examination of the gas within, one of the electrodes being used for the other terminal of the exciting induction coil.
"The three electrodes were constructed as is shown in detail in Fig. [4.2]. B was the electrode itself, made of... # 20 tungsten wire. This was firmly sealed directly through the Pyrex walls in the manner shown, for mechanical strength. The entire surface of the electrodes was first covered with a thick layer of Pyrex glass, A. The tip was then carefully ground off until the tungsten was exposed. Then a hole, C, was drilled in the end with a # 80 drill, 0.343 mm in diameter, the hole being less than 0.76 mm deep, to receive the fine wire for explosion. The electrodes were then sealed into the bulb. This method of sealing in the electrodes had the two purposes of excluding the chance of leakage of air inward through the seal after evacuation and of preventing the liberation of gas from these electrodes by the heating effect of the explosion itself. With such electrodes only the surfaces of the three small holes were exposed to the effects of the explosions, and one of these, in the spectroscopic capillary, was far removed from the scene of the explosion. In some of the early explosions brass electrodes were used welded to a tungsten wire sealed through the glass.
"The bulb was vacuum-evacuated for 15 hours by a mechanical pump and two mercury-vapor diffusion pumps in series with a liquid-air trap to capture any mercury vapor. The bulb was supported in a furnace and heated to above 350o C to drive off any gases contained in the glass, and out-gassed coconut charcoal (immersed in liquid air) was employed inline to absorb gases just prior to sealing the bulb. In addition, about 0.2 amperes from a battery was passed through the electrodes and the filament to heat them above 2000o C for 15 hours to drive off any other absorbed gases. Bulbs prepared in this manner showed no spectrum, florescence, or conductance."
After the wire was exploded, spectroscopic analysis of the gas revealed the strong yellow line of helium, and the faint green line of mercury. Other faint lines were detected but not identified: two red, one bright blue, and one pale violet. On some occasions, two unidentified faint yellow lines and a second violet line were detected. Hydrogen and neon were absent. Wendt and Irion commented:
"The appearance of helium and the absence of hydrogen is interesting for two reasons. In the first place, it seems to dispose of the objection that the helium arose from gas remaining in the wire, for in that case hydrogen should also have been visible, for it was probably originally present in the wire in much larger quantity than was helium. In the second place, if the helium does arise from a decomposition of the tungsten atoms, the absence of hydrogen is also interesting because the atomic weight of tungsten is exactly 46 times the atomic weight of helium, and Rutherford was also unable to detect hydrogen from the bombardment with a-rays of carbon, oxygen, magnesium, silicon, and sulfur, whose weights are multiples of 4, though he did detect it with boron, nitrogen, fluorine, sodium, phosphorus and aluminum, whose weights are not such multiples." (2, 3)
The possibility that helium could have been present in the tungsten could have been excluded by exploding the wire using a greater inductance to obtain a slower explosion at a lower temperature, giving complete vaporization without decomposition. However, there was not enough time available to conduct such tests. The vacuum method of preparing the tubes rigorously excluded contamination, but did not allow the collection, measurement and analysis of the gas produced. Therefore, Wendt and Irion also conducted explosions in carbon dioxide at atmospheric pressure in slightly modified bulbs; this enabled them to study the helium they produced. The carbon dioxide was carefully purified and blank-tested. This method also excluded the possibility of contamination from leakage of air into the bulbs, or by the release of gas from the glass bulb or the electrodes, because the explosion was too rapid to liberate any helium from those sources by heat from the tungsten vapor. The brief duration of the high temperature could not cause the carbon dioxide to decompose into carbon monoxide and oxygen, and the scientists performed pertinent tests to prove the point.
Unfortunately, the Associated Press widely published an exaggerated account of the "transmutation" experiment, based on the oral presentation which Wendt and Irion had made to the American Chemical Society in April, 1922. In a footnote to their article published in the Journal of the ACS (September 1922), they emphasized that, "this report is preliminary, and that nothing is proved beyond the importance of the problem and the promise of this method... For the sake of clarity it is suggested that the term disintegration be reserved for the spontaneous processes of radio-activity, that decomposition be applied to the splitting of complex atoms into simpler parts, and that transmutation be understood to imply some degree of synthesis of atomic nuclei."
Wendt and Irion planned a compete analysis of the gas they collected, but the sample was lost in an accident. "Then the work was stopped by the failure of health of the senior author..." Two years later, S.K. Allison and William Harkins reported inconclusive negative results from their version of the experiment. Scientific American magazine, however, sponsored a test of the experiment and published the successful results! The issue remains unresolved to this day. (1)
1. Allison, S.K. & Harkin, William D.: J. Amer. Chem. Soc.. 46 (4): 814-824 (April 1924) "The Absence of He from the Gases left after the Passage of Electrical Discharges: I, Between Fine Wires in a Vacuum; II, Through Hydrogen; III, Through Hg Vapor"
2. Rutherford, Sir Ernest: Nature 109 (2735): 418 (1 April 1922); "Disintegration of Elements”"
3. Rutherford, E.: Science 55 (1425): 422-423 (21 April 1922); "Disintegration of Elements"
4. Wendt, Gerald L. & Irion, Clarence E.: J. Amer. Chem. Soc. 44 (9): 1887-1894 (September 1922); "Experimental Attempts to Decompose Tungsten at High Temperatures"
5. Wendt, G.E.: Science 55 (1430): 567-568 (21 April 1922); "The Decomposition of Tungsten"
Figure II. 4. 1
Figure II. 4. 2