54 POPULAR SCIENCE MONTHLY. 



circuits of the receiving and transmitting stations to be put in com- 

 munication so that they have the same electrical time-period.* 



In the Cantor Lectures before the Society of Arts in 1900, on elec- 

 trical oscillations and electric waves, the author has discussed at length 

 the conditions under which powerful electrical oscillations can be set 

 up in a circuit. It was there shown that every electric circuit having 

 capacity and inductance has a particular or natural time-period of elec- 

 trical oscillation depending on the product of these qualities, and that, 

 to accumulate powerful electrical oscillations in it, the electromotive 

 impulses on it must be delivered at this rate. Illustrations were dra'WTi 

 from mechanics, such as the examples furnished by vibrating pendu- 

 lums and springs, and from acoustics, as illustrated by the phenomena 

 of resonance, to show that small or feeble blows or impulses delivered 

 at the proper time intervals have a cumulative effect in setting up 

 vibrations in a body capable of oscillation. It is a familiar fact that 

 if we time our blows, we can achieve that which no single blow, how- 

 ever powerful, can accomplish in throwing into vibration a body such 

 as a pendulum, which is capable of oscillation under the action of a 

 restoring force. Precisely the same is true of an electric circuit. We 

 have already seen that the receiving aerial has an alternating electro- 

 motive force set up in it by the impact of the successive electric waves 

 sent out from the transmitter. It must, however, be remembered that 

 the transmitter sends out a series of trains of waves, not by any means 

 a continuous train, but one cut up into groups of probably ten to fifty 

 waves, each separated by intervals of silence, long, compared with the 

 duration of a single train of waves. 



If, however, by a suitable adjustment of capacity and inductance, 

 we make the natural time-period of oscillation of the receiving aerial 

 circuits agree with those of the transmitting aerial, within certain 

 limits the former will only be receptive for waves of the frequency sent 

 out by the transmitter. It is quite easy to illustrate this principle by 

 numerous experiments. It can be done by means of an apparatus de- 

 vised by Dr. Georg Seibt for showing in an interesting manner the syn- 

 tonization or tuning of two electric circuits. This consists of two bob- 

 bins, each consisting of one layer of insulated wire wound on a wooden 

 rod (see Fig. 22). Each of these bobbins has a certain electrical capacity 

 with respect to the earth, when considered as an insulated conductor, and 

 it has also a certain inductance. If therefore electromotive impulses 

 are applied to one end of the bobbin at regular intervals, electrical 

 oscillations will be set up in it, and, as already explained, if these are 



* The capacity of an electrical circuit corresponds to the elastic plia- 

 bility, or what is commonly called the elasticity, of a material substance, 

 and the inductance to mass or inertia. Hence capacity and inductance are 

 qualities of an electric circuit which are analogous to the elasticity and 

 inertia of such a body as a heavy spring. 



