Here's why:

* Electric field strength is defined as the force experienced by a unit positive charge placed at a point in the electric field.

* The unit of force is Newton (N), and the unit of charge is Coulomb (C).

* Therefore, the unit of electric field strength is N/C.

* Since 1 Volt (V) is equal to 1 Joule (J) per Coulomb (C), and 1 Joule (J) is equal to 1 Newton-meter (Nm), we can express the unit of electric field strength as N/C = J/C/m = Vm.

So, a volt-meter (Vm) is a valid unit for electric field strength.