The practical unit of electric capacity; the capacity of a conductor which can retain one coulomb of electricity at a potential of one volt. The quantity of electricity charged upon a conducting surface raises its potential; therefore a conductor of one farad capacity can hold two coulombs at two volts potential, and three coulombs at three volts, and so on. The electric capacity of a conductor, therefore, is relative compared to others as regards its charge, for the latter may be as great as compatible with absence of sparking and disruptive discharge. In other words, a one farad or two farad conductor may hold a great many coulombs. Charging a conductor with electricity is comparable to pumping air into a receiver. Such a vessel may hold one cubic foot of air at atmospheric pressure and two at two atmospheres, and yet be of one cubic foot capacity however much air is pumped into it. The farad is equal to one fundamental electrostatic unit of capacity multiplied by 9E11 and to one electro-magnetic unit multiplied by 1E-9. The farad although one of the practical units is far too large, so the micro-farad is used in its place. The capacity of a sphere the size of the earth is only .000636 of a farad. [Transcriber's note: Contemporary calculations give about .000720 farad.]