RADIOTELEGRAPHY—FLEMING. 169 
Tf, however, the oscillations are damped, then we see in the mirror 
a train of images each decreasing in length (fig. 9, pl. 1). On apply- 
ing such an oscillograph vacuum tube to the circuit of an inductively 
coupled antenna, and examining in a revolving mirror the image of 
the electrodes, they will be seen to present an appearance as in fig. 10, 
pl. 1, taken from photographs kindly given me by Herr Hans Boas, 
of Berlin. These oscillograms indicate that there are two oscillations 
present of different frequency, producing an effect similar to beats 
in music. Owing to the difference in frequency, the oscillations 
alternately reenforce and extinguish each other throughout the period, 
and as this type of oscillogram is only obtained with an inductively 
coupled antenna, it is a proof that in such a case there are two 
oscillations present of different frequencies. A similar result has 
been obtained by Prof. E. Taylor Jones with low-frequency oscil- 
lations in coupled inductive circuits by means of an electrostatic 
oscillogram of his own invention. Looking at these photographs, it 
will be seen that each represents a single train of damped oscillations 
gradually dying away, but that in each train of oscillations there is 
an alternate waxing and waning of the amplitude, which indicates 
that it may be considered to be composed of two superimposed oscil- 
lations of different frequency (fig. 10a, pl. 1). 
Accordingly, in the case of wireless telegraph antenne inductively 
coupled, we have in general two waves radiated of different lengths, 
and either of these can be made to affect suitably tuned receiving 
circuits. These waves have different damping and different maximum 
amplitudes. 
One of the disadvantages of close inductive coupling is, therefore, 
that we must divide the energy given to the antenna between two 
waves of different length. As the receiving antenna is generally only 
tuned to one of these wave lengths, we then capture and absorb only 
the energy conveyed by the waves of that wave length. To meet this 
difficulty it has been the custom to employ a feeble coupling between 
the circuits of the oscillation transformer, so as to generate waves 
of only one wave length. The objection then arises that the energy 
conveyed to the antenna is much reduced. It is, however, possible, 
as I have shown, to duplicate the receiving circuits so as to capture 
the energy of both the waves even with close coupling of the trans- 
mitter transformer ¢ (see fig. 11). 
A method of creating feebly damped oscillations has, on the other 
hand, recently been developed, generally known in Germany as Wien’s 
@Since the delivery of this lecture my attention has been drawn by Mr. J. 
Hettinger to an article by himself in the ‘‘ Hlectrical Engineer” of October 26, 
1906, in which he describes an almost identical arrangement devised by him for 
capturing both the waves of an inductively coupled transmitter, and refers to a 
prior invention for the same purpose by Dr. G. Seibt. 
