APPENDIX A: MODIFICATION OF 
L. J. ANDERSON’S COMPENSATION METHOD 
FOR THERMISTOR BEADS 
When a number of temperature sensors are used in an array, 
it is desirable for all the sensing elements to have the same vari- 
ation in electrical parameter with temperature. In the instrument 
described here, the temperature sensor is the thermistor. The 
thermistor is a semiconductor device with a large negative tem- 
perature coefficient of resistance. The resistance, #, of a therm- 
istor as a function of temperature is given empirically by the 
expression 
b 
R=ae /T (Al) 
where @ and / are constants and 7 is the absolute temperature. 
The constants, aq and), vary from thermistor to thermistor, and 
the fit of the empirical expression to the thermistor characteris- 
tics may vary. This appendix discusses the means by which the 
resistance vs. temperature characteristics of the several therm- 
istors may be compensated to produce one common character - 
istic. 
L. J. Anderson* developed one technique for thermistor 
beads. In his method, the resistance of each bead of a group is 
determined precisely at two temperatures, zi and 7 5 Let the 
corresponding resistances for a typical bead be rand ro Two 
resistance values, 7,and #,, are chosen. R, is chosen equal to 
the greatest thermistor resistance, Tans in the group, and fi, is 
chosen equal to the lowest thermistor resistance, 7, (fig. Al). 
For each thermistor, a resistance, ), is placed in series, and a 
resistor, a, is placed in parallel. The values are so chosen that 
the net circuit resistance be fie abr. and Fi, at ie . The formulas 
given by Anderson for these values are: 
1 
2 
ae e = EviGualgs 
Beige POE) BOE Gee Fae, (A2) 
*Reference Al, page 55 
33 
