Page:Popular Science Monthly Volume 63.djvu/117

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discharges it through a galvanometer. If this galvanometer is subsequently standardized, so that the ampere value of its deflection is known, we can determine easily the capacity C of the aerial or insulated conductor, reckoned in microfarads, when it is charged to a potential of V volts, and discharged n times a second through a galvanometer. The series of discharges are equivalent to a current, of which the value in amperes A is given by the equation,

A = nVC105

and hence if the value of the current resulting is known, we have the capacity of the aerial or conductor expressed in microfarads, given by the formula,

C = A106nV

A series of experiments made on this plan have revealed the fact that if a number of vertical insulated wires are hung up in the air and rather near together, the electrical capacity of the whole of the wires in parallel is not nearly equal to the sum of their individual capacities. If a number of parallel insulated wires are separated by a distance equal to about 3 per cent, of their length, the capacity of the whole lot together varies roughly as the square root of their number. Thus, if we call the capacity of one vertical wire in free space, unity, then the capacity of four wires placed rather near together will only be about twice that of one wire, and that of twenty-five wires will only be about five times one wire.


PSM V63 D117 Various forms of marconi antennae.png

Fig. 8. Various forms of Aerial Radiator. a, Single Wire; b, Multiple Wire; c, Fan Shape; d Cylindrical; g, Conical.


This approximate rule has been confirmed by experiments made with long wires one hundred or two hundred feet in length in the open air. Hence it points to the fact that the ordinary plan of endeavoring to obtain a large capacity by putting several wires in parallel and not very far apart is very uneconomical in material. The diagrams in Fig. 8 show the various methods which have been employed