In the schematic below, I have a 741 in an inverting configuration.
Hi Carl,
1) Please show the complete schematics, including power supply and other components (i.e. capacitors on the supply rails etc).
2) The way you are trying to measure the output impedance is wrong for this circuit. The output impedance is quite low and if you "measure" it by increasing the load, the output will be overloaded well before you get a meaningful result.
3) The correct way to measure the output impedance of an amplifier like this one is to supply a small AC current which would not overload the output (say, 1mA RMS) directly to the output (and ground the input), for example, using 10V RMS signal from a generator and 10K resistor. The AC voltage you measure on the output pin will be proportional (1mV/Ohm for 1mA current) to the output impedance providing that impedance is much smaller than 10K. Unfortunately, you need a sensitive AC millivoltmeter or at least an oscilloscope capable of 1-2 mV/div gain to measure this low level AC signal. Or you can use another amplifier - say, x100 gain and AC coupled, connected to the output of your 741 and than even your multimeter may be able to measure the amplified voltage reasonably accurately.
Cheers
Alex