the coil is capable of an extremely rapid vibration.  The potential may be raised by increasing, as far as the experimenter judges proper, the rate of change of the primary current.  With a coil not widely differing from the present, it is best to connect the two primaries in multiple arc; but if the secondary should have a much greater number of turns the primaries should preferably be used in series, as otherwise the vibration might be too fast for the secondary.  It occurs under these conditions that misty white streams break forth from the edges of the discs and spread out phantom-like into space.  With this coil, when fairly well produced, they are about 25 to 30 centimetres 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.