If slime is kept inside a sealed container and the humility of the air is low the slime mould will gradually dehydrate and become inactive.
Filling the bottom of the container with layer of (non-nutrient) agar and adding nutrients (say oat flakes) maintains good humidity and food source but makes electrical measurements difficult as the agar (which is mostly water) is electrically conductive.
Adding small blobs of (non-nutrient) agar inside the container will help to maintain suitable humidity but they can dry out over time.
Adding medium sized blobs of agar will maintain the humidity for longer and keep the slime active.
Adding large blobs of agar will keep the slime active for longer but care has to be taken to ensure the blobs don’t touch/merge and form an electrical ‘short circuit’ across the electrodes.