THE L3 SYSTEM — QUALITY CONTROL IN MANUFACTURE 991 



cially available composition type resistors due to their high temperature 

 coefficient and lack of stability. Fortunately a new unit, known as a 

 boro-carbon resistor was under development and could be utilized. The 

 boro-carbon films employed in this unit have a maximum temperature 

 coefficient of resistance of 0.01 per cent per °C compared to ordinary 

 deposited carbon films which may have an average temperature coeffi- 

 cient of 0.03 per cent per °C. 



Two types of boro-carbon resistors covering approximately 45 re- 

 sistance values are being produced for the L3 system. These types are 

 the 200A resistors, held to a tolerance of ±3 per cent and the 200B re- 

 sistor held to a tolerance of ±1 per cent. Both types are available in 

 resistance values from 5 ohms to 9999 ohms inclusive. Of the 45 values 

 used in L3 equipment, approximately 40 are of the 200A type and 5 are 

 of the more precise 200B type. 



3.42 Manufacturing Procedure 



The core of the 200-type resistor consists of a high grade alkaline earth 

 porcelain of special composition. The resistance film is produced by plac- 

 ing the cores in a heated furnace containing a reducing atmosphere of 

 hydrogen and injecting a combination of a hydro-carbon gas and boron 

 trichloride into the furnace. Subsequently, cracking occurs depositing a 

 thin film of boro-carbon over the surfaces of the cores. The resistance 

 value of the film thus formed is dependent on the relative gas concentra- 

 tions and the duration of exposure in the furnace. By suitable control, 

 blanks having a resistance range from 5 to 250 ohms are produced. 

 After removal from the furnace a band of silver paste, consisting of silver 

 flake in a suitable binder, is applied to each end of the coated ceramic 

 and baked. These bands form the contact surfaces for the terminals. 



The resistors are next sorted into resistance groups preparatory to 

 adjustment to the desired resistance value. Resistors having values be- 

 tween 35 ohms and 50 ohms are used to produce the higher resistance 

 values between 250 ohms and 9999 ohms. The resistors falling outside 

 the 35 to 50 ohm range are held for simple adjustment by rubbing. 

 The method used to obtain higher resistance values is to cut a helix 

 through the carbon film using a diamond cutting wheel. Resistance values 

 may be raised by as much as 1000 times by proper selection of the pitch 

 of the spiral groove. 



After helixing, these resistors together with those initially outside the 

 35 ohm to 50 ohm range are fitted with a lead assembly at each end. Final 

 adjustment of the resistance value is obtained by abrading the entire 



