| amplifier | <microscopy> When the projected image of the microscope is projected less than 250 mm the objective may be moved enough from its usual position (for visual observation) to degrade the image. It can be restored by adding a lens above the ocular to bring the image into focus on the desired plane but with the objective still in its position of best-corrected image. A similar system requires an ocular in which the same corrections are made. Such lens systems are termed amplifiers. (05 Aug 1998) |
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| amplifier host | A host in which infectious agents multiply rapidly to high levels, providing an important source of infection for vectors in vector-borne diseases. (05 Mar 2000) |
| image amplifier | A device for converting a low light level fluoroscopic image to one that can be seen by the eye in a lighted environment; usually consists of an electronic light amplifier chained to a television tube. Synonym: image intensifier. (05 Mar 2000) |
| two-stream amplifier | <radiobiology> Microwave amplifier based on the two-stream instability. (09 Oct 1997) |
| vacuum | Origin: L, fr. Vacuus empty. See Vacuous. 1. <physics> A space entirely devoid of matter (called also, by way of distinction, absolute vacuum); hence, in a more general sense, a space, as the interior of a closed vessel, which has been exhausted to a high or the highest degree by an air pump or other artificial means; as, water boils at a reduced temperature in a vacuum. 2. The condition of rarefaction, or reduction of pressure below that of the atmosphere, in a vessel, as the condenser of a steam engine, which is nearly exhausted of air or steam, etc.; as, a vacuum of 26 inches of mercury, or 13 pounds per square inch. Vacuum brake, a kind of continuous brake operated by exhausting the air from some appliance under each car, and so causing the pressure of the atmosphere to apply the brakes. <medicine> Vacuum pan, a glass tube provided with platinum electrodes and exhausted, for the passage of the electrical discharge; a Geissler tube. Vacuum valve, a safety valve opening inward to admit air to a vessel in which the pressure is less than that of the atmosphere, in order to prevent collapse. Torricellian vacuum. See Torricellian. Source: Websters Dictionary (01 Mar 1998) |
| vacuum casting | The casting of a metal in the presence of a vacuum. (05 Mar 2000) |
| vacuum curettage | Aspiration of the contents of the uterus with a vacuum curette. (12 Dec 1998) |
| vacuum desiccator | A desiccator that can be evacuated. (05 Mar 2000) |
| vacuum disk phenomenon | The appearance of a radiolucent stripe in an intervertebral disk, a manifestation of disk degeneration; a misnomer since there is gas present. (05 Mar 2000) |
| vacuum extraction, obstetrical | Removal of the foetus from the uterus or vagina at or near the end of pregnancy with a metal traction cup that is attached to the foetus' head. Negative pressure is applied and traction is made on a chain passed through the suction tube. (12 Dec 1998) |
| vacuum extractor | Device for producing traction upon the head of a foetus by means of a soft cup held by a vacuum. (05 Mar 2000) |
| vacuum flask | <apparatus> A glass vessel, often silvered, with two walls, the space between which is evacuated; used for maintaining materials at constant temperature or, more usually, at low temperature. Synonym: vacuum flask. (05 Mar 2000) |
| vacuum headache | Headache due to closure of the frontal sinus. (05 Mar 2000) |
| vacuum pan | A device for growing crystals from solutions by gradually lowering the pressure within the sealed container holding the liquid. (09 Oct 1997) |
| vacuum tube | A glass tube from which the air has been removed, containing two or more electrodes, between which passes an electrical current or spark; used in the production of X-rays, or to control circuits. Previously in wide use, the vacuum tube has been supplanted by transistors in electronic circuits. (05 Mar 2000) |