ballast tube

ballast tube

[′bal·əst ‚tüb]
(electricity)
A ballast resistor mounted in an evacuated glass or metal envelope, like that of a vacuum tube, to reduce radiation of heat from the resistance element and thereby improve the voltage-regulating action.
McGraw-Hill Dictionary of Scientific & Technical Terms, 6E, Copyright © 2003 by The McGraw-Hill Companies, Inc.
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The machine uses forced air to accelerate molding sand up through an inside ballast tube, where it is scrubbed against sands trapped under a steel receptacle.