obtained by making the radiating spheres very small. The shortest wave-length produced is about 6 mm., and the corresponding number of oscillations is about 50,000 millions in a second. The frequency of vibration in this case will be seen to be about thirteen octaves lower than that which produces visible radiation. The intensity of radiation in the above case is rather feeble, and I use in general electric waves of about half an inch in length.
Fig. 1.—The Radiator.
The jointed electrodes carrying the beads are in connexion with a small modified Ruhmkorff's coil, actuated by a small storage cell. The usual vibrating interrupter is generally a source of trouble; the contact points get worn out and the break becomes irregular. The great objection (as Hertz found) to the continuous production of secondary sparks is the roughening of the surface of the radiating ball, by which the spark ceases to be oscillatory. It is very troublesome, in the middle of an experiment, to be obliged to take out the radiator for polishing. The flash of radiation produced by a single break is enough for an experiment, and it is a mere waste to have a series of useless oscillations. In my apparatus for quantitative measurements I have therefore discarded the vibrating interrupter in favour of a simple break-key. To economize space, I wind the condenser (a long strip of paraffined paper with tin foils on opposite sides) round the secondary of the coil, appropriate connexions being made with the interrupting key. The coil and a small storage cell are enclosed in a metal box, in accordance with the precautions which Prof. Lodge had found to be necessary. I used tinned iron in order to screen the space outside from magnetic disturbances due to the making or breaking of the primary circuit of the coil. A sudden magnetic variation disturbs the receiver. The iron box is placed inside a second box of thick brass or copper. These precautions are taken to prevent straying of electric radiation. Through a small opening in the back or side of the box the stud of the press-key projects. In front of the box is the radiator-tube, which may be square or cylindrical. Inside this tube is mounted the radiating originator. A flash of electric radiation is produced by a proper manipulation of the interrupting
