Original sentence: He developed the Heaviside step function and used to model the current in an electric circuit.Heaviside employed the vector calculus to reformulate these equations into the form by which they have been known ever since (see Maxwell's equations).Maxwell's formulation of electromagnetism required 10 equations
Edited sentence: He invented the Heaviside step function and employed it to model the current in an electric circuit.Heaviside employed the curl and divergence operators of the vector calculus to reformulate these 20 equations into four equations in four variables (B, E, J, and ρ), the form by which they have been known ever since (see Maxwell's equations).Maxwell's formulation of electromagnetism consisted of 20 equations in 20 variables