8/11/2019 Lamp Exhausting Process

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Lamp Exhausting Process

The fabrication of Cooper-Hewitt lamps was carried out completely by manual glassblowing operationhile this alone is a fascinating topic, of greater interest is the unique method by which the lamps were

and dosed with mercury.

hen ready for exhausting the lamps would be charged with about twice the required quantity of mercu

ung vertically in an upright gas furnace or a hot air oven. The lamp was attached to an ordinary vacuu

ia a small glass exhaust tube affixed to the upper end of the lamp just beside the iron anode. A plain racuum pump, which could evacuate the tube to a pressure of around 0.1 torr, was entirely suitable for

operation. A large mercury trap was placed in-line between the lamp and pump to prevent mercury tran

ack into the pump.

The ovens would then be ignited, gradually raising the temperature of the glass to outgas it and also brin

ercury slowly up to its boiling temperature. The relatively heavy mercury vapour would then begin to

from the reservoir at the bottom of the lamp, and like a piston, would push all air, water vapour and forep to the top of the tube and out of the lamp. As the temperature continued to rise and the glass was bro

erilously close to its softening temperature, the mercury would begin to boil vigorously and the lamp i

urned into a highly efficient mercury vapour diffusion pump, producing its own high vacuum devoid oolatile 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 toolts, further heating the boiling mercury cathode and rendering the iron anode incandescent at white-h

ensure that it was thoroughly outgassed.The fabrication of Cooper-Hewitt lamps was carried out comp

anual glassblowing operations, and while this alone is a fascinating topic, of greater interest is the uni

ethod by which the lamps were exhausted and dosed with mercury.

hen a measured quantity of mercury had been boiled out of the lamp and collected in the trap, the proould be stopped and the exhaust tube quickly melted over while all was still hot, thus sealing the rema

he mercury inside the lamp under ultra-high vacuum. Thereafter the pressure inside the lamp is related

he cold spot temperature of the mercury - some 357C in operation, the boiling point of mercury under

acuum. Because of this limitation the pressure inside Cooper-Hewitt lamps rarely ever rose above 1 ton operation.