that thingy's pocket bulb is eggshell thin, I should concider it only for distillations without vacuum applied.
And if you use it at all, be damn carefull when you insert a thermometer into it, so you don't cause a crack in the bottom.
Especially when you want to fill it with a suitable heat conducting fluid, think first what this specific fluid would do to your motherliquid boiling under it, when the bulb should burst.
The heat conducting fluid needs to be having a much higher boiling point than your motherfluid, or it will start to boil up and away out of the pocket during distilling.
Then it will also be of less harm if the pocket bulb bursts and the h.c.fluid hits the boiling motherfluid in the distillation flask, contrary to f.ex. an alcohol used, which would cause a nifty vulcano outburst through the broken thermopocket.
Using pure mercury as heat conducting fluid would be shear madness btw, if that perhaps came to anyones mind.
All in all, I would advice to only use it for atmospheric distillations of simple solvents, and not for oils with a high boiling point, far over 100°C, such as MDP2P etc., where a narrow boiling trajectory is of great importance to your expected yield.
Use eventually silicone oil as the pocket filling. That has a very high boiling point. Or peanut oil.
Better stick to a ground glass-connector thermometer. They are not so overly expensive.
Most of the time you also need a ground connector fitting your wide neck progressing to the commercially available smaller ground-neck thermometers.
Btw, I always used a small diameter screwed glass connector on top which progresses to a wide ground connector, fitting in my flask necks, and a special high temp screw-on PPP thingy, with a silicone rubber ring in it, through which my long thermometers can be sticked through, so I can adjust, by screwing it's mutter tight, my thermometer bulb just into the boiling motherfluid in my distillation flasks. Works perfect for even the highests vacuums. And can be adjusted on the fly, when the fluid level in the flasks lowers, while it's distilling away. LT/