Resistance below zero

Hello. We got resistance below zero during short time(~10sec) when measured one of samples and after 10-20sec we got resistance more zero and equal to ~80GOm;.Could you please explain reason of resistance below zero?<br> <br> Sample: sintered Al2O3 (density &gt;99,99%), thickness=500µm;<br> Device: Keithley 6517b;<br> Settings:&#160;<br> Speed = HiAccurancy;<br> Ampsrel = Disable;<br> Damp = Enable;<br> Vsource = Manual;<br> Meas-type = Resistance;<br> Amperage = 20mA;<br> Voltage = 500V;<br> Filter = Averaging(25RDGS);<br> +<br> with store like Resistance&#160;by Time;<br> (and could you please dont lock my account because I cant connect/call to your support)

Is the sample in any sort of shielded test fixture?<br> <br> Double check your connections.&#160; Are you making use of meter-connect to connect LO of Amps meter to the LO of the V-Source?<br> <br> When was your electrometer last calibrated? Perhaps try with an axial leaded high ohms device as practice and to raise confidence in function of the instrument.<br> <br> There are many instructional videos on the 6517B web page.<br> Try this one:&#160;&#160;https://www.tek.com/en/video/how-to/how-to-make-a-resistance-measurement-using-the-model-6517b-electrometer-with-auto-and-manual-modes