and property. But the practical conditions require not only the
judicious determination of the dimensions of the apparatus; they
likewise necessitate the employment of energy of the proper kind. It
is easy, for instance, to construct a transformer capable of giving,
when operated from an ordinary alternate current machine of low
tension, say 50,000 volts, which might be required to light a highly
exhausted phosphorescent tube, so that, in spite of the high
potential, it is perfectly safe, the shock from it producing no
inconvenience. Still, such a transformer would be expensive, and in
itself inefficient; and, besides, what energy was obtained from it
would not be economically used for the production of light. The
economy demands the employment of energy in the form of extremely
rapid vibrations. The problem of producing light has been likened to
that of maintaining a certain high-pitch note by means of a bell. It
should be said a _barely audible_ note; and even these words would not
express it, so wonderful is the sensitiveness of the eye. We may
deliver powerful blows at long intervals, waste a good deal of energy,
and still not get what we want; or we may keep up the note by
delivering frequent gentle taps, and get nearer to the object sought
by the expenditure of much less energy. In the production of light, as
far as the illuminating device is concerned, there can be only one
rule--that is, to use as high frequencies as can be obtained; but the
means for the production and conveyance of impulses of such character
impose, at present at least, great limitations. Once it is decided to
use very high frequencies, the return wire becomes unnecessary, and
all the appliances are simplified. By the use of obvious means the
same result is obtained as though the return wire were used. It is
sufficient for this purpose to bring in contact with the bulb, or
merely in the vicinity of the same, an insulated body of some surface.
The surface need, of course, be the smaller, the higher the frequency