Voltage Drop to a Readable signal

[Instro_Spark]

Hey just wondering if someone would be able to help me measure voltdrop across a resister. At the moment im trying to measure the voltage drop with a voltage divider setup. Im actually using a trim pot so ive connected the two end pins across the resister and then measured the voltage of the wiper pin.

What i cant understand is that when i measure with my multimeter across the resistor i get a range from 1-5v but when the trim pot is connected it gives me a reading from 23v to 19v

So how would i go with getting this high reading to a much smaller signal
Ive attached a schematic below (sense resister is R10)

Circuit details
+5v
+24v
0-10v input signal converts to a 4-20ma output which allows me inject a current into a loop powered instrumentation device
Handful of opamps

[KJDS]

The good news is that the circuit is doing what I would expect it to do.

To get it to do what you want it to do, then connect one end of the 250R resistor to one op-amp input and the other end to the other input. Make these connections via a pair of 100k resistors to half the voltage, so that +24V becomes +12V at the op-amp, then set it up for a gain of x2. That way you avoid being outside the input common mode voltage range.