662 Professor J. A. Fleming [June 4, 



coupled syntonic electric currents. If one circuit has free oscillations 

 set up in it, the action and reaction of the circuits generates oscilla- 

 tions of two frequencies. Accordinijly, when an antenna circuit is 

 coupled to a condenser circuit, we have oscillations of tAvo frequencies 

 set up in it, and waves of two wave-lengths radiated from the 

 antenna. The presence of these two waves can be detected either by 

 measurements made with the cymometer or by an oscillograph 

 vacuum tube. In the first case all that is necessary is to place a 

 cymometer in proximity to the antenna and vary its oscillation con- 

 stant. It will be found that there are two settings of the handle for 

 Avhich the Neon tube glows brightly, and the scale of the instrument 

 will indicate the wave-lengths of the two waves respectively. Some 

 instructive measurements of this kind have been made by Professor 

 W. G. Pierce in a recent research, and he has shown that the wave- 

 length given by the formula which can be deduced from the theory 

 of the operations are in agreement with actual measurements (see 



Ked Pcnduluir. 



Blue Pendulum 

 Fig. 7. 



Fig. 8). Another striking confirmation can be obtained by the 

 oscillograph vacuum tube, invented by Dr. Gehrcke of the Reichsan- 

 stalt, Berlin. This consists of a glass tube having two strip electrodes 

 in it nearly touching, which are made of nickel or aluminium. 

 The tube is filled with pure nitrogen and exhausted to a pressure 

 of about 10 to 20 mm. If such a tube has a high voltage applied 

 to its terminals, a glow light extends along the electrodes, the 

 length of which varies with the electromotive force. Hence, if 

 the tube is connected to a circuit in which an oscillatory discharge is 

 taking place, the glow light along the tube will rapidly extend and 

 contract. If the electrodes are examined in a revolving mirror, 

 making from fifty to a hundred turns a second, the images of the 

 glowing electrodes corresponding to each oscillation will be separated 

 out, and if the oscillations are persistent or undamped, we see a series 

 of short bright lines alternately above and below a central line. 

 If, however, the oscillations are damped, then we see in the mirror 

 a train of images each decreasing in length (see Fig. 9). On apply- 



