Eight hours is not long enough to properly calibrate a hygrometer. I assume the one that came with the humidor is an analog one. Those are for decorative purposes only and fairly inaccurate.
By an "electric hygrometer", am I correct in assuming you mean a digital battery powered one?
Those that have larger format humidors make use of electricity to run a humidifier and have the RH readings built in. But all smaller format humidors use a removable type of hygrometer.
This thread should help.
http://www.cigarasylum.com/vb/showthread.php?t=619
Also, how big is your humidor? What is the advertised capacity?