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BELL SYSTEM TECHNICAL JOURNAL 



could be shaped to definite dimensions and contacts applied. These meth- 

 ods include: 1) melting the semiconductor, cooling and solidifying, cutting 

 to size and shape; 2) evaporation; 3) heating compressed powders of semi- 

 conductors to a temperature at which they sinter into a strong compact 

 mass and firing on metal powder contacts. While all three processes have 

 been used, the third method has been found to be most generally useful 

 for mass production. This method is similar to that employed in ceramics 

 or in powder metallurgy. At the sintering temperatures the powders 

 recrystallize and the dimensions shrink by controlled amounts. The powder 

 process makes it possible to mix two or more semiconducting oxides in 

 varjnng proportions and obtain a homogeneous and uniform solid. It is 

 thus possible to cover a considerable range of specific resistance and tem- 



Fig. 1. — Thermistors made in the form of a bead, rod, disc, washer and flakes. 



perature coefficient of resistance with the same system of oxides. By '' 

 means of the powder process it is possible to make thermistors of a great 

 variety of shapes and sizes to cover a large range of resistances and power 

 handling capacities. 



Figure 1 is a photograph of thermistors made in the form of beads, rods, 

 discs, washers and flakes. Beads are made by stringing two platinum alloy 

 wires parallel to each other with a spacing of five to ten times the wire diam- 

 eter. A mass of a slurry of mixed oxides is applied to the wires. Surface 

 tension draws this mass into the form of a bead. From 10 to 20 such beads 

 are evenly spaced along the wires. The beads are allowed to dry and are 

 heated slightly until they have sufficient strength so that the string can be 

 handled. They then are passed through the sintering furnace. The oxides I 

 shrink onto the i)latinum alloy wires and make an intimate and permanent, I 

 electrical contact. The wires then are cut to separate the individual beads. i( 



