Ive always had trouble with mesh current method for some reason, I was following the all about circuits page http://www.allaboutcircuits.com/vol_1/chpt_10/3.html And reading how setting the current directions were arbitrary. So I re-tried the problem for the wheatstone bridge basically using completely random current directions, and I could not get the mesh currents to equal the same as the answer at the bottom of the page

heres a scan of my sign conventions/equations

Can someone please tell me what im doing wrong? id really appreciate it.
thanks

 

The first thing I see that's wrong is in the first equation.

You have:

150(I1+I3) + 50 I1 + 100(I1-I3) = 0

but it should be:

150(I1+I3) + 50 I1 + 100(I1-I2) = 0

For the second equation you have:

300(I2-I3) + 100(I2-I1) + 250 I2 = 0

but it should be:

300(I2+I3) + 100(I2-I1) + 250 I2 = 0

After collecting terms for the second equation, you have a term 550 I2; it should be 650 I2.

 

ooh! thanks !