332 Professor Oliver Lodge [June 1, 



persistent vibrator,* and, therefore, well adapted for interference and 

 diffraction experiments. But, indeed, spheres can be made to vibrate 

 longer than usual by putting them into copjDer hats or enclosures, in 

 which an aperture of varying size can be made to let the waves out, 

 Figs. 20 and 21. 



Many of these senders will do for receivers too, giving off sparks 

 to other insulated bodies or to earth ; but, besides the Hertz type of 

 receiver, many other detectors of radiation have been employed. 

 Vacuum tubes can be used, either directly or on the trigger principle, 

 as by Zehnder, Fig. 13,*(" the resonator spark precipitating a discharge 

 from some auxiliary battery or source of energy, and so making a 

 feeble disturbance very visible. Explosives may be used for the 

 same purpose, either in the form of mixed water-gases or in the form 

 of an Abel's fuse. Fitzgerald found that a tremendously sensitive 



Fig. 13. 



Zehnder's Trigger Tube. Half natural size. The two right-hand ter- 

 minals, close together, are attached to the Hertz receiver ; another 

 pair of terminals are connected to some source just not able to make 

 the tube glow until the scintilla occurs and makes the gas more 

 conducting — as observed by Schuster and others. 



galvanometer could indicate that a feeble spark had passed, by reason 

 of the consequent disturbance of electrical equilibrium which settled 

 down again through the galvanometer. J This was the method he 

 used in this theatre four years ago. Blyth used a one-sided electro- 

 meter, and V. Bjerknes has greatly developed this method, Fig. 14, 

 abolishing the need for a spark, and making the electrometer metrical, 

 integrating and satisfactory. § With this detector many measurements 

 have been made at Bonn by Bjerknes, Yule, Barton, and others, on 

 waves concentrated and kept from space dissipation by guiding wires. 



Mr. Boys has experimented on the mechanical force exerted by elec- 

 trical surgings, and Hertz also made observations of the same kind. 



Going back to older methods of detecting electrical radiation, we 

 have, most important of all, a discovery made long before man 



* J. J. Thomson, ' Eecent Researches/ 344. f Wied. Ann., xlvii. p. 77. 

 % Fitzgerald, ' Nature,' vol. xli. p. 295, and vol. xlii. p. 172. 

 § Wied. Ann., xliv. p. 74. 



