I think what you are looking for is an op-amp set up as a subtractor:

I think what you are looking for is an op-amp set up as a subtractor:

Edit:

The above is the simplest form of a differential op-amp, however, it is best to use instrumentation amplifier (wikipedia) setup as mentioned in the comments. It is basically the same concept, but with the use of two extra op-amps (that works as buffers) you can prevent the input signals to be interferred by the load, giving you a much more acurate reading.

Here is the instrumentation amplifier setup:

Do a little reading on the link above, it is fascinating how they work.