Once you measure a_1 and a_2 for the two isotopes, you can then take their ratio a_2/a_1 = (µ_2 * I_1)/(µ_1 * I_2). This equation stems from the assumption that adding extra neutrons to the nucleus does not affect the electron cloud. This assumption then comes from the fact that neutrons have zero charge and thus do not interact with the electrons via the Coulomb potential. Although neutrons are composed of quarks——which are inherently charged particles and can interact with electrons via the weak interaction——this is so small that it is negligible in this case. The only other thing that could effect the electron cloud is gravity, but this is also so small that it negligible. Thus, for this case, neutrons will not interact with electrons. Taking our equation above, we can then rewrite it to obtain µ_2 as follows: µ_2 = (a_2 * I_2 * µ_1)/(a_1 * I_1). Thus, we have determined µ_2 for the other isotope.