PYROLYTIC FILM RESISTORS: CARBON AND BOROCARBON 



303 



tion of atmospheric constituents is proportional to the enclosed volume of 

 air. Hence, where high temperature operation is not necessary, hermetical 

 sealing in air provides resistor units of relatively high stability free from 

 any hysteresis in their resistance-temperature characteristics. When the 

 carbon film is thick and the enclosed volume of air is small, such resistors 

 can also be operated at higher temperatures if the permanent increase in 

 resistance due to partial oxidation of the fibn and consumption of the en- 

 closed oxygen can be tolerated. 



6000 

 5500 

 5000 

 4500 

 4000 

 3500 

 3000 

 2500 

 2000 

 1500 

 1000 

 500 

 

 -500 



10 15 20 25 



30 35 40 45 50 55 

 ELAPSED TIME IN DAYS 



60 65 70 75 80 85 



Fig. 19 — Resistance aging of varnished and hermetically enclosed pyrolytic carbon re- 

 sistors as influenced by thermal cycling and time. 



The most stable pyrolytic carbon resistors are those in which the resistive 

 unit is sealed in an hermetical enclosure which is baked and evacuated 

 or filled with an inert gas. During the pumping and baking the resistance of 

 such a unit decreases due to removal of previously adsorbed gases and 

 residual low-molecular-weight hydrocarbons from the carbon film. Hence 

 in contrast to the varnish-coated units, which are adjusted below tolerance 

 before application of the finish, hermetically sealed units are adjusted to 

 values somewhat above tolerance prior to sealing. 



To increase thermal dissipation over that which obtains with the evacu- 

 ated unit, and to achieve rapid response of the resistor temperature to that 



