Wave-length of Electric Radiation by Diffraction Grating. 169 



measures its own wave-length. There is, however, one difficulty in 

 reconciling the theoretical value with that actually obtained. 

 According to theory, the wave-length should be equal to twice the 

 circumference, or 2?r times the diameter of the circular resonator. 

 The value actually obtained by Messrs. Sarasin and De la Rive is, as 

 has been said before, eight times the diameter of the circle. 



Rubens, using a bolometer and Lecher's modification of the slide 

 bridge, determined the nodes and loops in a secondary circuit in 

 which stationary electric waves were produced. A curve obtained 

 by representing the bolometer deflections as ordinates and the 

 distances of the bridge from one end as abscissae, shows the harmonic 

 character of the electric disturbance in the wire. It was found that 

 the wave-length obtained by this method did not depend on the 

 period of the primary vibrator; the wave-length measured was 

 merely that of the free vibration started in the secondary circuit by 

 the primary disturbance. 



Hertz's method is therefore the only one for the measurement of 

 electric waves in air, and the result obtained by this method is 

 vitiated by the influence of the periodicity of the resonator. It was 

 therefore thought desirable to obtain the wave-length of electric 

 radiation in free space by a method unaffected by any peculiarity of 

 the receiver. 



I have succeeded in determining the wave-length of electric 

 radiation by the use of curved gratings, and the results obtained 

 seem to be possessed of considerable degrees of accuracy. Rowland's 

 method of using the curved grating for obtaining diffraction light 

 spectra was also found well suited for the production of pure spectra 

 of electric radiation. The focal curve / in this arrangement is a 

 circle, having as a diameter the straight line joining the centre of 

 curvature C with the apex M of the grating. 



FIG. 1. 



Gr, the grating ; M, its apex ; f, the focal curve. 



A source of radiation situated on this curve will give a diffracted 

 spectrum, situated on the same curve defined by the equation 

 (a+ 6) (sin i sin 6) n\ 



