long. When the hand is held against them no sensation is produced, and
a spark, causing a shock, jumps from the terminal only upon the hand
being brought much nearer. If the oscillation of the primary current
is rendered intermittent by some means or other, there is a
corresponding throbbing of the streams, and now the hand or other
conducting object may be brought in still greater proximity to the
terminal without a spark being caused to jump.
[Illustration: FIG. 11.--PHANTOM STREAMS.]
Among the many beautiful phenomena which may be produced with such a
coil I have here selected only those which appear to possess some
features of novelty, and lead us to some conclusions of interest. One
will not find it at all difficult to produce in the laboratory, by
means of it, many other phenomena which appeal to the eye even more
than these here shown, but present no particular feature of novelty.
Early experimenters describe the display of sparks produced by an
ordinary large induction coil upon an insulating plate separating the
terminals. Quite recently Siemens performed some experiments in which
fine effects were obtained, which were seen by many with interest. No
doubt large coils, even if operated with currents of low frequencies,
are capable of producing beautiful effects. But the largest coil ever
made could not, by far, equal the magnificent display of streams and
sparks obtained from such a disruptive discharge coil when properly
adjusted. To give an idea, a coil such as the present one will cover
easily a plate of 1 metre in diameter completely with the streams. The
best way to perform such experiments is to take a very thin rubber or
a glass plate and glue on one side of it a narrow ring of tinfoil of
very large diameter, and on the other a circular washer, the centre of
the latter coinciding with that of the ring, and the surfaces of both
being preferably equal, so as to keep the coil well balanced. The
washer and ring should be connected to the terminals by heavily
insulated thin wires. It is easy in observing the effect of the
capacity to produce a sheet of uniform streams, or a fine network of
thin silvery threads, or a mass of loud brilliant sparks, which
completely cover the plate.
Since I have advanced the idea of the conversion by means of the
disruptive discharge, in my paper before the American Institute of
26