Mass corresponds to inductance, stiffness to elastance and damping to resistance. One advantage is that an increase in elastance increases impedance. The terms "elastance" and "elastivity" were coined by Oliver Heaviside in 1886. Electrical elastance is the inverse of capacitance. The tidal volume varies depending on the resistance and elastance of the respiratory system. The SI unit of elastance is the inverse farad (F). The SI unit of elastance is the reciprocal farad (F). The total elastance is simply the sum of the individual elastances in that case. Thus, an elastance can be defined in any energy domain. The slope of the voltage-stored charge curve is called "differential elastance" in this field. Elastance is also used in microwave engineering. The reciprocal of capacitance is called elastance. However, elastance does still see some usage by theoretical writers.