48 
increases. If the value of series resistor, r , becomes very 
large, the bridge becomes essentially a constant current device. 
These are the conditions when Kaufman* talks about large values 
of series resistors in half-bridges in connection with linear input 
devices such as platinum thermometers. 
EXAMPLE 
Table B1 gives the average resistance of five glass bead 
thermistors (VECO 32A1) at eleven different temperatures. These 
five thermistors have been compensated (see Appendix A) so that 
they all have very similar resistance-versus-temperature char- 
acteristics (the maximum deviation from the values stated below 
for any of the thermistors was only 1.3 ohms). The thermistors 
were calibrated in the NEL controlled temperature tank, which 
is capable of holding any desired temperature to within +0.002°C** 
TABLE Bl. AVERAGE RESISTANCE OF GLASS 
BEAD THERMISTORS. 
Temperature, °C Resistance, ohms 
) 3781.2 
10 3640.0 
11 3504.4 
12 3374.0 
13 3248.8 
14 BIL} 6) 
15 3013.7 
16 2903.3 
Wi 2796.5 
18 2694.2 
19 2596.4 
Let us compute the value of the linearizing series resistor, 
r, from eq. (B15). Let R, = 3640.0 ohms, fp = 3128.9 ohms, 
and Ra = 2694.2 ohms. Then 7 = 2687 ohms. Now obtain the 
value of ” using the method of Beakley:*** 
*Reference Bl, page 55 
**Reference B4, page 55 
***Reference B38, page 55 
