I have a wall wart rated at 12vdc output that I measured with a quality dvm before using on a model railroad project but after unplugging from the AC source I (for absolutely no logical reason).. measured again unplugged and still read 12.4 vdc output I of course questioned my meter and measured it again with a quality Hewlett Packard. dvm and got the same solid measurement? Huh????....How is that possible? I measured it unplugged again 24 hours later and now measure 8.2vdc solid without ever plugging the transformer in again?? It basically acts like a slowly degrading battery. I thought of stored capacitance but that makes no sense since loading it with the meter should fully disipate that immediately. This defys any logic I can come up with?? Any viable theory out there?