Drops of water coming out of nozzles are charged plus or minus depending on the charge carried by nearby electrodes, which are connected to collecting screens on the other side. There is always some initial charge that may be plus or minus to start the process. Charge builds up exponentially until at about 6000V a small spark jumps between the two spheres in front of the meter.
Briefly press buttons 1, 2 and 3 in succession to operate. There may be a 1-minute delay while the water is recirculated. Apparatus shuts off after 40 seconds; restart by pressing 3.