Its great radiating power damps it down very rapidly, so that it does not make above two or three swings; but nevertheless, each time it is excited, sparks can be drawn from most of the reasonably elongated conductors in the theatre of this Institution, and indeed from wire fencing and iron roofs outside this building.
A suitably situated gas leak can be ignited by these induced sparks. An Abel’s fuse connecting the water pipes with the gas pipes will blow off; vacuum tubes connected to nothing will glow (this fact has been familiar to all who have worked with Hertz waves since 1889), electric leads, if anywhere near each other, as they are in some incandescent lamp-holders, may spark across to each other, thus striking an arc and blowing their fuses. This blowing of fuses by electric radiation frequently happened at Liverpool till the suspensions of the theatre lamps were altered. They had at first been held in position by wire guides, which served as collectors of the Hertz waves or impulses.
Fig. 10.— Hertz Oscillator on reduced scale,
⅒th inch to a foot.
The striking of an arc by the little reverberating sparks between two lamp-carbons connected with the 100-volt mains I incidentally now demonstrate. An arc is started directly the large Hertz vibrator is excited at a distance.
There are some who think that lightning flashes can do none of these secondary things. They are mistaken.
Specimens of Emitters and Receivers.
On the table are specimens of various emitters and receivers such as have been used by different people; the orthodox Hertz radiator ([Fig. 5]), and the orthodox Hertz receivers:—A circular ring ([Fig. 6]) for interference experiments, because it is but little damped, and a straight wire for receiving at a distance, because it is a much better absorber. Beside these are the spheres and ellipsoids (or elliptical plates), which I have myself introduced and mainly used ([Fig. 19]), because they are powerful radiators and absorbers, and because their theory has been worked out by Horace Lamb and J. J. Thomson. Also dumb-bells ([Fig. 11]) without air gap, which must be excited by a positive spark at one end and a negative spark at the other, and many other shapes, the most recent of mine being the inside of a hollow cylinder with sparks at ends of a diameter ([Fig. 12]); this being a feeble radiator, but a very persistent vibrator,[6] and, therefore, well adapted for interference and diffraction experiments. But, indeed, spheres can be made to vibrate longer than usual by putting them into copper hats or enclosures, in which an aperture of varying size can be made to let the waves out (Figs. [20] and [21]).
Fig. 11.— A Small Dumb-bell Form of Radiator for Impulsive Rush.