Wheatstone bridge with 741 op amp HELP!

hi , new to this site so hi to everyone!
i have a wheatstone bridge connected to an op amp

I have designed this circuit to measure resistance of paper. R1 is noted as a varible resistor - this is acting as my probes. It is 14k ohms as that is the base line for my paper and also to balance the bridge.
The problem i have is that every time i vary R4 the gain of the amp changes?!
Im sure i am doing something stupid somewhere.
can anyone point me in the right direction please?
thankyou

R4 is connected parallel to RA so of course changing its value also changes the gain of the opamp.
You need an intrumentation amplifier or a differential amplifier made with 3 opamps.

hi,
Download the datasheet for the AD623 instrumentation amplifier from Datasheet & Application Note Database, PDF, Circuits, Datasheets | Datasheet Archive

You dont need an instrumentation amplifer; a simple rail-to-rail output amplifier (which a 741 most certainly is not) configured this way will let you set the gain without effecting the offset. I centered the response curve by making the lower resistors in the bridge 14K instead of 2K, but it works with the 2K resistors, too.

In the plot, the X-axis is the sensor resistance. The slope of the output is a function of the gain resistor, without effecting the zero point.

The second sim shows that you can change the zero point without effecting gain.

Thankyou all very much for your help. im now going to plug it into multisim and have a go.