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.
The problem, therefore, of long-distance telegraphy by Hertzian waves
is largely, though not entirely, a matter of associating sufficient
energy with the aerial wire or radiator. There are obviously two
things which may be done; first, we may increase the capacity of the
aerial, and secondly, we may increase the charging voltage or, in
other words, lengthen the spark gap. There is, however, a well-defined
limit to this last achievement. If we lengthen the spark gap too much,
its resistance becomes too great and the spark ceases to be
oscillatory. We can make a discharge, but we obtain no radiation. When
using an induction coil, about a centimetre, or at most a centimetre
and a half, is the limiting length of oscillatory sparks; in other
words, our available potential difference is restricted to 30,000 or
40,000 volts. By other appliances we can, however, obtain oscillatory
sparks having a voltage of 100,000 or 200,000 volts, and so obtain
what Hertz called "active sparks" five or six centimetres in length.
Turning then to the question of capacity, we may enquire in the next
place how the capacity of an aerial wire can be increased. This has
generally been done by putting up two or more aerial wires in
contiguity and joining them together, and so making arrangements
called in the admitted slang of the subject "multiple aerials." The
measurement of the capacity of insulated wires can be easily carried
out by means of an appliance devised by the author and Mr. W. C.
Clinton, consisting of a rotating commutator which alternately charges
the insulated wire at a source of known electromotive force and then
discharges it through a galvanometer. If this galvanometer is
subsequently standardised, 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 = (nVC) / (10^{6}),
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