Eudiometers are basically glass containers with electrodes going into them. Between the electrodes you can create a spark.
This example has only one electrode and this is insulated from the armature. The spark goes off between the electrode and the armature. It is caused by the combination of gases within the bottle, usually ordinary air and "inflammable metallic air" i.e. hydrogen.
Alessandro Volta normally used a eudiometer to test the breathability of air.