192 



BELL SYSTEM TECHNICAL JOURNAL 



directly proportional to the current. This fluctuating potential is called 

 noise and since it depends on the current it is called current noise. In order 

 to obtain the best signal to noise ratio, it is necessary that the current noise 

 at operating conditions be less than Johnson or thermal noise.'^ ■* To make 

 noise-free units it is necessary to pay particular attention to the raw mate- 

 rials, the degree of sintering, the grain size, the method of making contact 

 and any steps in the process which might result in minute surface cracks or 

 fissures. 



POWER IN WATTS 



0.1 I 10 



THERMISTOR ELEMENT CURRENT IN MILLIAMPERES 



100 



Fig. 16. — Logarithmic plots of voltage versus current for six values of heater curren 

 in an indirecth' heated thermistor. Resistance and power scales are given on the diag 

 onal lines. 



All the thermistors discussed thus far were either directly heated by the 

 current passing through them or by changes in ambient temperature. In 

 indirectly heated thermistors, the temperature and resistance of the thermis- 

 tor are controlled primarily by the power fed into a heater thermally coupled 

 to it. One such form might consist of a 0.038 centimeter diameter bead of 

 Material No. 2 embedded in a small cylinder of glass about 0.38 centimeter 

 long and 0.076 centimeter in diameter. A small nichrome heater coil hav- 

 ing a resistance of 100 ohms is wound on the glass and is fused onto it with 

 more glass. Figure 16 shows a plot of log V versus log / for the bead ele- 

 ment at various currents through the heater. In this way the bead resist- 

 ance can be changed from 3000 ohms to about 10 ohms. Indirectly heated 

 thermistors are ordinarily used where the controlled circuit must be iso- 

 lated electrically from the actuating circuit, and where the power from the 

 latter must be fed into a constant resistance heater. 



