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Internal resistance again.



1) Sorry for spelling errors in my last message.

2) I am using a d.c. power supply to pass a
current through a Ti foil. From the positive of the
power supply the current goes through the
ammeter and through the foil (firmly squeezed
between two metallic plates). The voltage is
measured between these plates. I expected
voltage to be I*R but it is not.

I=1 A --> 0.75 volts (R=0.75 ohms)
I=2 A --> 1.10 volts. (R=0.55 ohms)

If anything I would expect R to go up with I, due
to higher temperature. In reality the temperature
does not change (to a touch) significantly.

Two days ago I used an Al foil and observed
similar results (same power supply).

I=1.3 A --> 0.55 volts
I=2.0 A --> 1.06 volts
I=2.9 A --> 1.30 volts

How can this be explained? I am sure that R
is essentially constant. The impedance of the
voltmeter is certainly much larger than 1 ohm.
Looks like the "internal r" of my power supply
changes a lot with the current. Why is it so?
Ludwik Kowalski