We may next consider more particularly the energy which is available for radiation and which is radiated. In the original form of simple Marconi aerial, the aerial itself when insulated forms one coating or surface of a condenser, the dielectric being the air and ether around it, and the other conductor being the earth. The electric energy stored up in it just before discharge takes place is numerically equal to the product of the capacity of the aerial and half the square of the potential to which it is charged. If we call C the capacity of the aerial in microfarads, and V the potential in volts to which it is raised before discharge, then the energy storage in joules E is given by the equation, E = (CV^{2}) / (2 · 10^{6}). Since one joule is nearly equal to three-quarters of a foot-pound, the energy storage in foot-pounds F is roughly given by the rule F = (3/8)CV^{2}/10^{6}. For spark lengths of the order of five to fifteen millimetres, the disruptive voltage in air of ordinary pressure is at the rate of 3,000 volts per millimetre. Hence, if S stands for the spark length in millimetres, and C for the aerial capacity in microfarads, it is easy to see that the energy storage in foot-pound is F = (27CS^{2}) / 8. If the aerial consists of a stranded wire formed of 7/22 and has a length of 150 feet, and is insulated and held vertically with its lower end near the earth, it would have a capacity of about one three ten-thousandths of a microfarad or 0·0003 mfd.[6] Hence, if it is used as a Marconi aerial and operated with a spark gap of one centimetre in length, the energy stored up in the wire before each discharge would be only one-tenth (0·1) of a foot-pound. By no means can all of this energy be radiated as Hertzian waves; part of it is dissipated as heat and light in the spark, and yet such an aerial can, with a sensitive receiver such as that devised by Mr. Marconi, make itself felt for a hundred miles over sea in every direction. This fact gives us an idea of the extremely small energy which, when properly imparted to the ether, can effect wireless telegraphy over immense distances. Of course, the minimum telegraphic signal, say the Morse dot, may involve a good many, perhaps half-a-dozen, discharges of the wire, but even then the amount of energy concerned in affecting the receiver at the distant place is exceedingly small.