From the above definitions it is apparent that if a solid-state semiconductor 
is substituted for control resistor Ry (fig. ID) then this substituted resistance 
will vary the current flow in the meter or recorder according to its resistance 
change, which is dependent on the environment. Rj, instead of being a 30,000-ohm, 
manually operated, variable potentiometer, then becomes an environmentally operated 
resistance. Usefulness of such environmentally controlled circuits to the insect 
ecologists is multifold. 
Solid-state, resistance-control mechanisms are classified according to the 
environmental factor upon which they are dependent, and also according to physical 
properties. They are here grouped into the following categories: 
Photoconductive Cells.--Photoconductive cells are cells in which the electric- 
al resistance varies inversely with the intensity of light that strikes the active 
Substance. Many such cells are commercially available today. They are sometimes 
called light-dependent resistors (LDR) and are usually made of thin layers of 
selenium, silicon, cadmium sulfide (CdS), thallus sulfide, galena crystal, lead 
telluride, and lead sulfide. Such cells require a supplementary voltage source, 
and have good infrared sensitivity and high signal-to-noise ratio. They do not, 
however, have the excellent frequency response of vacuum phototubes (Mark, 1956). 
Moisture is also a major inhibitor of photoconductors, and destroys conductivity 
(Zmuda, 1962). The best known solid-state photoconductor is the cadmium sulfide 
cell (CdS), used today in many camera light meters. These cells have reached a 
high degree of development within the last few years, and are excellent light- 
control mechanisms. Below 5000 R, the photoconductivity of CdS remains constant 
down to 2500 A. But, unlike most photoconductors, infrared illumination tends to 
quench the photoconductivity above 5200 &R. Such cells are also Poe sensi- 
tive detectors of X-rays and corpuscular radiation. A transport of 10~ electrons 
has been demonstrated for a single X-ray quanta (Zworykin and Ramberg, 1949). 
Photovoltaic cells.--Photovoltaic cells are cells capable of generating a 
voltage when exposed to light radiation. This category includes the selenium 
cells now used in such diverse applications as colorimeters, lighting controls, 
and relay circuits (Sasuga, 1962). Another type of cell that fits the photo- 
voltaic definition more closely than selenium cells is the silicon or solar cell, 
commonly used in banks as battery cells for satellites. A silicon cell can 
convert sunlignt directly into electricity and thus, when used with a highly 
sensitive relay or meter, requires no supplementary power source. 
Thermistors.--Thermistors are substances in which the electrical resistance 
varies inversely with the temperature. They are solid-state semiconductors formed 
from ceramic materials made by sintering mixtures of metallic oxides, such as 
manganese, nickel, cobalt, or uranium (Thermistor Manual, Anonymous, 1962). Thern- 
istors can be used either to control or measure temperatures, and they attain con- 
siderable advantage over ordinary thermocouples in that neither polarity nor lead 
length affects their operation; also, no reference temperatures or cold junction 
compensations are required, as is the case with thermocouples. Their high sensi- 
tivity, fast response, and small size make them extremely useful to entomologists 
concerned with measuring and controlling microenvironment of small insect cages. 
The thermistor's wide resistance change per degree change in temperature produces 
excellent accuracy of measurement; i.e., 78 ohms per degree centigrade compared 
with only 7.2 ohms for a platinum resistance bulb (Anonymous 1962). 
alt= 
