Consider ${{\rm{I}}_1}$ and ${{\rm{I}}_2}$ are the currents flowing simultaneously in two nearby coils 1 & 2, respectively. If ${{\rm{L}}_1} = $ self inductance of coil $1,{{\rm{M}}_{12}} = $ mutual inductance of coil 1 with respect to coil 2, then the value of induced emf in coil 1 will be