The spark counter consists of a tightly stretched wire beneath a metal gauze which will detect nuclear radiation. When a radioactive source is brought near the gauze, the air between it and the wire below becomes ionised making it more conductive and therefore sparks find it easier to jump the gap. Teachers may use this alongside or instead of the Geiger counter to produce a more impressive demonstration.
The spark counter needs to be attached to an EHT unit via high voltage leads and the working voltage is around 4KV although models vary so check the recommendations which came with the unit. Keep increasing the voltage from zero until a spark is seen beneath the gauze, then turn down just slightly. Bring a source near, keeping the end away from your body and sparks should be visible every couple of seconds or so depending upon the strength of the source.
Dust will gather under the gauze over time. Usually the gauze and lid can be removed for cleaning. Ensure the wire is taut at all times and no damage to the casing or gauze can be seen before each use.