Digits on arrows must sum to the digit in the connected circle.
On thermos digits must increase from the bulb-end.
The thermo in box #5 is not needed for a unique solution, but provides for a smoother solution path.
Solution code: the middle row, from left to right.
on 16. February 2024, 22:52 by thespeak
Great puzzle! Each step kept me on my toes. I agree with Dennis about the thermo in Box 5. I'm pretty sure that it smoothly solves without either of the short, two cell thermos and stays smooth throughout.
on 16. February 2024, 19:16 by dennischen
Fantastic stuff, fwiw I think it is perfectly smooth without the box 5 thermo.