Micro-ducers.--Micro-ducers are solid-state transducers which vary in 
resistance over a wide range with a change in force or pressure. Rare earths are 
processed with zirconium tetrachloride to produce resins, which undergo a change 
in resistance with a change of pressure (Hefferline et al., 1960). Resistance 
changes are not affected by temperature variations from -20° to 300° F., and the 
transducers' reliability is not impaired by continuous usage. Clark Electronics 
teformmertioctl! produce a pressure-sensitive paint from which hundreds of pressure 
transducers can be made for experimental purposes. They are applicable to 
measurements in the fields of motion, speed, density, vacuum, or pressure, and 
can operate relays and circuits without amplification. They will operate from 
either a.c. or d.c. voltage. 
Circuits 
Inexpensive recording and control circuits, that have been successfully used 
in studying ecology and behavior of the corn earworm, are shown in figures 1 to 
6. Variations or modifications of these circuits are found in the references at 
the end of this paper, and entomologists will no doubt devise many new uses. It 
is recommended that researchers interested in instrumentation obtain the publi- 
cations listed (many of which are free from the manufacturers) and make up an 
instrumentation looseleaf handbook. 
The series circuit (fig. 1A) may be converted to a simple thermistor 
circuit by the substitution of the appropriate thermistor for the manual variable- 
control potentiometer R,. The battery, thermistor, and microammeter (Ro) or 
microamp-recorder form @ simple series circuit. The thermistor may be mounted 
at extreme distances from the circuit and recorder; e.g., up among the leaves of 
a tree. The thermistor should be of high resistance, preferably above 100,000 
ohms. Because of the high resistance, as long as the voltage remains constant, 
the current flow will be determined only by the thermistor temperature, and any 
change in the resistance of a long transmission line due to ambient temperature 
will be negligible (Anonymous, 1962). The author and Mr. V. J. Valli of the 
U.S. Weather Bureau have used this type of circuit to continuously record temper- 
atures in the corn silk channel where the corn earworm feeds. 
A bridge circuit may be constructed to form an even more sensitive 
temperature-measuring circuit (Taylor, 1952). Besides temperature measurements, 
the Thermistor Manual (Anonymous, 1962) lists the following diverse uses for 
simple thermistor circuits: Temperature compensation, temperature control, 
liquid level measurements, time-delay circuits, remote control, switching, power 
measurements, voltage control, altimeters, and thermal-conductivity instruments. 
Thermistors come in many sizes and shapes, and are thus easily fitted into 
various insect-cage designs. Disks, washers, beads, glass probes, rods, matched 
pairs, and probe assemblies are a few of the many configurations obtainable 
(fig. 8). Most manufacturers stock mounting hardware, and mount configurations 
for various types of thermistors. 
Since the resistance of the thermistor is a function of its absolute temper- 
ature, excessive electrical current will heat it above the ambient temperature, 
causing a resistance drop.: For this reason, thermistors are tested with very 
minute currents so that there will be no measurable increase in the thermistor 
temperature. The values obtained are designated by the mathematical expression 
Ro» and Ro values are usually given at a specified degree centigrade in the 
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