I am studying physics and I am reading a guide to the franck-hertz-experiment and I am a little bit confused about the +/- notation of the potential $U_A$ and $U_B$.
The experiment is divided in two parts. In the first one we want to measure the current $I_A$ (electrons, who arrive at the anode). These electrons are accerlated by $U_B$.
First question: To accelerate the electrons, $U_B$ has to be positive ($U_B > 0$) (corresponding to the picture which I've added), right?
To break the electrons, $U_A$ is also $>0$ because the +/- in the picture is switched. Correct?
Now where the confusion began:
To measure the ionisation energy of Hg the manual want me to put a constant negative voltage on $U_A$ (so $U_A < 0$) to keep the electrons away from the anode and provide that all possibly created ions arrive at the anode. But using a negative voltage $U_A < 0$ would mean that the electric field lines would show in the different directions, right?
With $U_B > 0$ and $U_A < 0$ there would be just a complete acceleration in y-direction, wouldn't it?
I couldn't find any answer to solve my confusion. I hope someone can help me out. Maybe its a notation error in the manual or I didn't understand the experiment.
What I thought: To measure the ionisation-energy, we increase $U_B$ until we measure a current $I_A$ (caused by positive ions)...