Electric field strength is measured in volts per meter (V/m), while magnetic flux density (magnetic field strength) is measured in volt-seconds per square meter (Vs/m^2). Electric fields and magnetic fields are three-dimensional fields. It makes sense to measure a magnetic field in terms of a cross-section to find the density of the field, but it doesn't make sense to measure an electric field one-dimensionally. Is the per-meter referring to to an individual ray of light? Also, is V/m used to measure the strength of things like radio frequencies and X-rays?