I think that it might just be a very small capacitor (probably the whole thing is just a home made cap). Unless I'm not thinking this through right, it could simply be that without the 'device' the voltage spikes too quickly for the analog volt meter to show it, whereas with a tiny capacitor, it gets charged and holds the voltage long enough for the volt meter to display it. Being a tiny capacitor would also explain it dropping in voltage so quickly, as voltmeters usually make crap all difference to capacitors, and there is nothing else to cause the charge to drop (there is a diode stopping the coil completing the circuit).
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