In the context of this trial, the term judges refered to the jurors who were chosen by lot for civic service. There was no presiding judge like today. Each took an oath to make judgment according to the laws of Athens.
The jurors volunteered. Their names were chosen by lot for random service over a one year period. They were called judges as there was no presiding judge. Like today, jurors were paid, but the expectation was that one should really do jury duty as a service to the community (thus, like today, the compensation was modest). Nevertheless, many jury volunteers were old men who lacked other means of livelihood. Each took an oath to make judgment according to the laws of Athens.
The allotment machine at the right was used to select jurors for trials.