affected by temperature. Design considerations and 
temperature effects are discussed in the Zener Diode 
Handbook, pp. 24-27.'* To further reduce temperature 
effects, low-temperature coefficient resistors are used in 
the regulator, and low-temperature coefficient resistance 
wire is used for the sensor slidewire. 
Because of its flexibility, phonopickup wire is used 
for the connections to the sensor slidewire. Because 
these connecting wires have an appreciable resistance, 
there is a voltage drop across them as well as across the 
slidewire. The span adjust resistor, R?, in the voltage 
regulator is needed to make the voltage drop across the 
slidewires exactly equal to the span of the recorder. 
The zero adjust resistor, Ra, in each sensor gives the 
positive signal terminal the same electrical potential as 
the left-hand side of the slidewire, R3 (fig. 7). This 
causes the signal from the sensor to be zero when the 
contact is at the left-hand end of the slidewire, enabling 
full use of the recorder span. All sensors using the same 
regulator should have slidewires and connecting wires of 
the same length. 
CALIBRATION 
The sensor should be calibrated in the laboratory 
before being used in the field. In our calibration setup, 
the support rod was clamped rigidly in a horizontal 
position to a vertical rod with a laboratory clamp. The 
clamp wire was attached to the support arm of a small 
photographic enlarger that had a vernier arrangement for 
precise raising and lowering. A pointer attached to the 
support arm and a millimeter scale mounted on a vertical 
mirror just behind the pointer (to avoid parallax error 
during reading) allowed accurate measurement of height 
changes. A series of recorder millivolt readings were 
made at 1-mm. increments of height. A plot of height 
versus millivolt output tested the linearity of the sensor, 
and the slope of the straight line thus obtained was the 
1 International Rectifier Corp., Zener diode handbook. El 
Segundo, Calif. 74 pp. 1961. 
calibration constant of the sensor in terms of height per 
millivolt output (fig. 8). The linearity of the sensor 
depends on the accuracy of the lathe work in turning 
and centering the disk and axle, as well as on the 
uniformity in diameter and composition of the slidewire. 
It is not difficult to construct a sensor of excellent 
linearity and reproducibility. Good linearity of the 
sensor output permits direct determinations of growth 
rates simply by measuring the slope of portions of the 
recorder trace without need to consider position on the 
recorder scale. 
At the time of calibration, the sensor disk should be 
marked at 1-mv. intervals to provide a dial reading. In 
the field, a routine comparison of the sensor dial reading 
with that of the recorder is valuable to detect poor 
connections in the wiring or trouble with the recorder. 
