Bird Technologies 8141 - Manual (Page 13)

Chapter 2
Theory Of Operation
General
The Models 8141 and 8141-010 Termaline Load Resistor
consists essentially of a cylindrical film type resistor im-
mersed in a dielectric coolant. The resistor, individually se-
lected for its accuracy, is enclosed in a special tapered
housing which provides a linear reduction in surge imped-
ance directly proportional to the distance along the resistor.
This produces the uniform, practically reflectionless line ter-
mination over the stated frequencies of the load.
The dielectric coolant is chosen for its desirable electrical
properties and thermal characteristics. Cooling of the load is
accomplished by natural fluid and air convection. The 0.35
gallon (1.3 liter) of dielectric coolant carries the electrically
generated heat from the resistor to the walls of the cylindri-
cal cooling tank. The tank is encased in a set of heavy gauge
aluminum radiator fins, which are firmly pressed on the cyl-
inder. The heat from the dielectric oil is transferred to the
surrounding air through the fins by natural convection.
A synthetic rubber diaphragm located in the rear dome of
the load allows for the expansion of the coolant as the tem-
perature rises.
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