You can see from this crude diagram how the Leyden jar is charged
with the PVC pipe and paper towel sleeve. You try to hold the PVC
pipe as close as possible to the soda can electrode without knocking it
over. It takes a bit of practice to do this. You try to keep the pipe just a few
millimeters from the electrode.
The charging takes place on the forward stroke as the pipe
emerges from the sleeve. You should be able to hear a sound something
like tearing tissue as the Leyden jar charges, or even see some sparks
if the lighting is subdued. No charging takes place on the return stroke.
In fact, as the voltage builds up in the Leyden jar, some discharging takes place
into the paper towel because at these voltages, the paper is something of a conductor.
Therefore, when the voltage gets really high, it helps to bring the pipe close to the electrode
on the forward stroke and move it away from the electrode on the return stroke.
This is a refinement you probably won't need until you have had some practice at making
really long sparks.
The sparks that discharge between the
soda can and the ball bearings are quite bright and don't require subdued
light to be seen easily. It's best to start with a gap of about 2cm between
the the electrodes to get a feel of what it takes to get the setup to discharge
a spark. Up to about 8cm, the sparks will discharge spontaneously.
Beyond that, the sparks must be triggered.
To get longer sparks, you space the electrodes at the distance you want and then charge the Leyden jar to the point that it begins to hiss and spit from corona discharge. This means that the shape and size of the soda can electrode just can't hold any more voltage. When the setup is in this condition, move the PVC pipe about a meter away and take a couple of strokes without discharging it into anything. Then bring the charged PVC pipe into the area between the electodes and this will trigger the spark. In other words, you can get the spark when you want it, rather than waiting for it to happen when it will.