I am doing an experiment to measure the charge to mass ratio of an electron. And I have found the method suggested by Thompson to account for the effect of the Earth's magnetic field[http://ajp.aapt.org/resource/1/ajpias/v58/i10/p1019_s1?isAuthorized=no" [Broken]]. But it only gives the equation, i.e. B_total=B_coils+B_earth, when the earth's B field is (anti)parallel to the field from the coils. How about when the earth's B field is oriented at an angle to the B field from the coils? Can you suggest journals or textbook references that mentions this case?