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The fabrication of Cooper-Hewitt lamps was carried out completely by manual glassblowing operations, and while this alone is a fascinating topic, of greater interest is the unique method by which the lamps were exhausted and dosed with mercury.
When ready for exhausting the lamps would be charged with about twice the required quantity of mercury, and hung vertically in an upright gas furnace or a hot air oven. The lamp was attached to an ordinary vacuum system via a small glass exhaust tube affixed to the upper end of the lamp just beside the iron anode. A plain rotary vacuum pump, which could evacuate the tube to a pressure of around 0.1 torr, was entirely suitable for the operation. A large mercury trap was placed in-line between the lamp and pump to prevent mercury transport back into the pump.
The ovens would then be ignited, gradually raising the temperature of the glass to outgas it and also bringing the mercury slowly up to its boiling temperature. The relatively heavy mercury vapour would then begin to rise up from the reservoir at the bottom of the lamp, and like a piston, would push all air, water vapour and foreign gases up to the top of the tube and out of the lamp. As the temperature continued to rise and the glass was brought perilously close to its softening temperature, the mercury would begin to boil vigorously and the lamp itself was turned into a highly efficient mercury vapour diffusion pump, producing its own high vacuum devoid of all volatile substances except the mercury itself, which was present in great excess.
Simultaneously an alternating current arc would be struck inside the lamp at a potential of some 4000 to 6000 Volts, further heating the boiling mercury cathode and rendering the iron anode incandescent at white-heat to ensure that it was thoroughly outgassed.The fabrication of Cooper-Hewitt lamps was carried out completely by manual glassblowing operations, and while this alone is a fascinating topic, of greater interest is the unique method by which the lamps were exhausted and dosed with mercury.
When a measured quantity of mercury had been boiled out of the lamp and collected in the trap, the process would be stopped and the exhaust tube quickly melted over while all was still hot, thus sealing the remainder of the mercury inside the lamp under ultra-high vacuum. Thereafter the pressure inside the lamp is related only to the cold spot temperature of the mercury - some 357°C in operation, the boiling point of mercury under vacuum. Because of this limitation the pressure inside Cooper-Hewitt lamps rarely ever rose above 1 torr when in operation.
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