When using a glass thermometer, make sure that the thermometer is inserted into the bung so that the red bulb of alcohol at the bottom of the thermometer is positioned at the point where the steam is coming up and changes direction to go into the main condensing chamber, (usually about 2cm from the bottom of the bung, but ot so far as to be touching your packing). If the thermometer goes in too far then a higher reading will result. If it is not in far enough a lower reading may result.
NOTE: Switching to a ‘digital thermometer’ will be easier to read, they are much harder to break, and give a faster more accurate reading!