# How to extract linear term from polynomial?

Hello! Trying to measure the magnetic field of the earth here. Basically, measured the deflection of the current carrying wire in the vertical plane. However, the sag of the wire due to heat overwhelmed the displacement due to the magnetic field. If I plot displacement of wire against current, I get a polynomial (looks like quadratic). I need to extract the linear part of this as this is equivalent to the effects of the magnetic field. Can I just find the equation of the polynomial and take the linear term of the polynomial equation? Or do I have to use some more complicated extrapolation method?