OCR Text |
Show especially by walking too near them in soft snow. The 100- ohm helix potentiometer used is a ten turn Borg Micropot with associated ten- turn dial reading from 000 to 999. Galvanometer shunt resistor R2 should be one- tenth of the internal meter resistance or less. A 2- cell flashlight bulb connected in series with the battery ( not shown in the diagram) will reduce the heavy current drain by the resistance wire and prolong battery life. The Wheatstone bridge of the settlement gauge may readily be adapted to the measurement of snow temperature, with thermistors as sensing elements. Thermistors are semi- conductor elements with a large negative temperature coefficient of resistivity. This rapid change of electrical resistance with temperature can easily be measured with the settlement test set. The circuit modifications for temperature measurement are illustrated in the schematic diagram. The side of the bridge representated by the resistance wire in the settlement measurements is replaced electrically by two separate elements, the thermistor and a resistor ( R2) whose value is approximately equal to the thermistor resistance at 0° C. This latter value is close to 3000 ohms for the Fenwall Type G378 thermistor currently in use. A thermistor is attached by short ( 2- 3 ft) lead wires to each settlement plate, and placed on the snow surface adjacent to the plate. The measured plate position thus indicates thermistor position, and permits calculation of temperature gradients in the snow cover. One lead from the thermistor is connected to the settlement plate clip wire, and the other lead is connected by a separate, parallel wire to a separate terminal strip in the junction box. After the settlement plate positions are read, the test set connecting plug is shifted to the temperature circuit, as shown in the diagram, and SW2 is then thrown to the temperature ( closed) position. The bridge is balanced for each - 2 - |