The only ones I've had have a little expansion area at the very top, where the bore is a larger diameter.
I've successfully heated up the bulb, such that the alcohol or mercury extends all the way up the tube, just into the expansion bit at the top.
When the fluid cools, it pulls the whole lot down again.
For obvious reasons, care is required so the contents don't expand too much and burst the glassware.

Bill

+1 for the idea of overheating to the extent that the level goes up into a larger diameter part of the glass tube, and I also see the risks.

I note that the OP's thermometer reads 0-50 degrees. If the thermometer is put in a freezer, in a vertical position, will the alcohol level drop into the bulb, and then expand in an unbroken column when the thermometer is returned to room temperature? Don't leave it in too long, you don't want the alcohol to freeze!

Rod

As above.

Last one I did was by over heating so the "bubble" went to the expansion area, (just dipped it in a hot kettle) easy to control – just take it out of the water to cool down.