We would need some very low resistance metals like 'gold', the whole arc could have been just one big battery.
Apparently, upon the chest were two cherubim of beaten gold, looking towards each other, and spreading their wings so that both sides of the propitiatory were covered. These could be positive and negative terminals.
Funny thing is the manna reference in the bible is sometimes attributed to mushrooms that Isrealites found when they were travelling to the promised land.

http://diseyes.lycaeum.org/fresh/mushmann.htm
BTW, please don't try eating strange fungi off the ground, or you might start seeing bright coloured lights, or even enjoy playing CA expansions that don't fix things like the enemy walking up to your castle walls without seige equipment and standing there whilst you shoot them.

Yes ok I'm in a funny mood.