Resistance measurement with a Keithley 2002, how to adjust current sourced

Hello, 
I am new to using the Keithley 2002 DMM and am using it to measure the resistance of a thermistor. The resistance  at room temperature (22 c) for the thermistor is R= 18,000 ohms.

My understanding is that when I use  the 20 kohm range (20,000 ohm) of the Keithley 2002 to measure the 18,000 ohm resistance, the electrical current sourced out by the Keithley 2002 is 89 uA (89E-6 A). This results in a power dissipated on this thermistor of P=i2xR=143 uWatts (1.43E-4 Watts).  

In my application I need to keep the power dissipated on the thermistor below 50 uWatts. I was wondering if there is a way to adjust the current sourced out by the Keithley 2002 to make it lower than 89 uA. Ideally lower the current down to 52 uA. 

Thank you!
Ron

A multimeter does not typically allow adjustable source current for the ohms measure function.

To achieve what you describe, you need a current source and then use the dmm to measure voltage to have insight about the R.
A SourceMeter such as 2450 can be the current source.  It might also serve as the voltage measure, but check the specs against what you need.

The 2002 is 8.5 digit instrument whereas the 2450 is 6.5 digit.