The spark counter is a series of thin wires a few millimetres above the surface of a silver metal disc. The potential difference is adjusted to a few thousand Volts. The electric field strength in the gap between the wires and the plate becomes so big that it forces the air to conduct and you get sparks. You get this to happen and then turn it down slightly so that it is almost bu not quite sparking. Then you bring in the alpha radioactive source, as shown in the picture. The sparks come back. But you get nothing with beta and gamma. This is because the alpha particles are able to ionise air molecules, allowing the air to conduct and producing sparks. Putting paper in the way stops it.
