The Y-arm is where the boiling point of the fractionating level is read and measured by a thermometer. The placement of the thermometer is crucial for an accurate boiling point reading. Too low and you’ll not be reading the temperature before the component has condensed and the reading will be too warm. Too high and the temperature reading will be after the mixture has condensed and so too cold. The bulb of the thermometer should be placed just below the point where the Y-arm stretches out to the condenser, as you can see in the figure below.
Figure 1: A distilling head with a thermometer. The bulb of the thermometer is just below the point where the Y-arm stretches out to meet the condenser.