increments. The wiper voltage was indeed the required linear function of rotor angle. Next the total  resistance was measured and found to be 6890 Ohm. Note that this was within the 6250 f 20% (5000 -  7500) Ohm tolerance allowed for this potentiometer.  When notified of this anomaly the manufacturer said that it was typical for the total resistance of a  conductive plastic pot to change after thermal cycling. The mechanism was explained as follows:  conductive plastic is made of carbon particles in an insulating matrix, co-cured at 160” C (and rated for  operation up to 125” C). Thermal cycles change the insulating matrix on a microscopic level, allowing  better conductivity between the carbon particles. Macroscopically these changes appear as a drop in the  total resistance. The manufacturer said that as the number of thermal cycles builds, the rate at which the  resistance drops will decrease and the resistance will eventually stabilize. The scratches and drops of  silver paint used to linearize the pot have an effect on the rate at which the resistance stabilizes. The  manufacturer assured me that the resistance changes would be uniform throughout the length of the  resistance element and that the pot’s linearity would not be affected by changes in the total resistance.  The eight flight WSA pots went through additional thermal cycling as part of the steering actuator  qualification test program. After those thermal cycles, the total resistances were found to have changed by  +73 to -283 Ohms. Another data point comes from the MIL-PRF-39023-style pots: between manufacture  and use, the total resistance of each one dropped from 200 to 400 Ohms.  From the beginning, WSA Potentiometer S/N 1039 was an outlier, with a total resistance 750 Ohms higher  than its nearest neighbor. When S/N 1039 was 5 weeks old, it took advantage of a thermal cycle to drop  its total resistance by 500 Ohms to a value more in line with, but still higher than, the rest of the batch.  While this drift was significant, 1039 has always been within tolerance, and testing proves its linearity.  Based on the preceding failure analysis, it was determined that there was no problem with the pot.  However, just to be safe, pot S/N 101 7 was labeled “Limited Use (Non-Flight Part)” and was not used for  flight.  LESSON 21. Drifting values and excessive variation within a lot may indicate trouble, even if the  individual pots are within spec. Compare the pots to each other as well as to the requirements.  Conclusions  As of this writing, the “Spirit” and ‘‘Opportunity’’ Mars Exploration Rovers have been operating on the  Martian surface for over two years. In spite of the difficulties encountered during flight qualification of the  MIL-PRF-39023-style pots, the HGAG pots and the WSA pots, all pots on the Rover continue to perform  reliably under very harsh conditions. It is hoped that the lessons provided in this paper will help other  potentiometer users enjoy similar successes.  References  1. Iskenderian, T. “Lessons Learned from Selecting and Testing Spaceflight Potentiometers.” Aerospace  Mechanisms Symposium #28, (1 994), pp. 339-358.  2. Defense Logistics Agency, “MIL-PRF-39023B Resistors, Variable, Nonwire-wound, Precision, General  Specification For.” Feb 1999.  3. “VRCI-P-1 OOA, Industry Standard for Wirewound and Nonwirewound Precision Potentiometers, Terms  and Definitions, Inspection and Test Procedures.” Variable Electronic Components Institute, Vista,  CA, http://www.veci-vrci.com/, 1 988  31 6