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Hi
Glass does conduct heat but poorly compared to other materials like steel and copper. Glass is not used in clinical thermometers for the purpose of increasing the sensitivity of thermometers (that means, how fast the thermometers respond to its surroundings).
Glass is used in clinical thermometers because it is transparent. Temperature can be read off quickly and easily from the thermometer.
If you notice, the glass wall around the bulb of the thermometer is very very thin, because glass is a not a material that conducts heat fast from the surrounding environment to the liquid (mercury or alcohol) in the bulb.
Note:
1. Sensitivity of a clinical thermometer depends on the liquid inside the bulb (either mercury or alcohol) and the bore diameter (the diameter of the 'tube' in the thermometer).
2. A side note, Glass is also used in other types of thermometers because it does not melt at high temperature. However the question was addressing clinical thermometers which does not detect high temperatures.
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