XLVIII. This plate shows one of the strongest discharges viewed similarly, as in the previous cases just dwelt upon. The streamers and spark issue again from the tip of the âfree terminalâ wire of the extra coil. Some of the sparks pass again in winding paths, which are very long, much longer than the straight course which they should take, to the top of the secondary. These paths have easily a length of some fifty feet or so and are of extreme brilliancy. This is partially due to the capacity of the ball of 30" diam. Connected to the top turn of the extra coil. Some very brilliant and thick sparks pass from the ball to the hood above and others much longer, though less brilliant, strike the high coil on the stand across the laboratory. One of the streamers is wonderfully interesting on account of the curiously twisted and curved appearance. It is hard to conceive how a discharge can pass through the air in this way when there exists a strong tendency to make it take the shortest route.
The curiously curved path clearly shows how extremely sensitive discharges of great length, and particularly those passing out into the air from a single terminal, are to currents of the air. This sensitiveness is still further increased when the streamer or spark is not compelled to issue always from the same point or points, as when the terminal is constituted by a pointed wire, but can issue with equal facility from other points as, for example, when the terminal is constituted by a large ball or disk. The slightest draught is, in such a case, sufficient to alter the position and shape of the streamer. In such an instance the discharge is also highly sensitive to other influences, as magnetic or electric actions, Roentgen rays, light and other forces or disturbances in the medium.
I now understand better why a ârotating brushâ, which I have described some years ago, is so wonderfully sensitive, many times more than any other sensitive device of which I have knowledge at present. It seems to me that a sensitive device in telegraphy on this principle will have to be ultimately adopted in preference to others. The trouble is that it is difficult, or at any rate inconvenient, to produce and maintain the phenomenon but this difficulty may be overcome in time. The photograph described shows very beautifully how a streamer falls apart and spatters after striking a wooden structure, owing to the sudden heat evolution and gas generation at that spot. The photograph conveys well the idea of the fierceness of the discharge produced by 100 throws of the switch, other particulars remaining.
