1909] 



071 Researches in Eadiotelegraphy . 



661 



arc. If the spark method is used, then the condenser is one of 

 relatively large capacity, and the inductance is kept small. If the 

 capacity is measured in electrostatic units, and the inductance in 

 electromagnetic units, the ratio of capacity to inductance may be 

 something of the order of 5 : 1 or even 20 : 1. In this case the 

 condenser is charged by means of an induction coil or transformer, 

 and discharged across a spark gap, and this discharge consists of 

 intermittent trains of electric oscillations with a periodic time equal 

 to the free natural period of the oscillatory circuit. These discharges 

 are made to succeed each other from 50 to 600 times a second, by 

 using an induction coil with an appropriate interrupter, or else an 

 alternator and a transformer. If the arc method of exciting the 

 oscillations is employed, then the ratio of capacity to inductance must 

 be much smaller and the oscillations are excited in this circuit by 

 a continuous current arc worked with a voltage from 200 to 400 volts 

 or more, the arc being traversed by a strong magnetic field and 

 generally being placed in a chamber kept free from oxygen. The 

 oscillations set up in the condenser 

 circuit are then persistent or un- 

 broken. The oscillations are ex- 

 cited in the antenna by coupling 

 it inductively or directly with the ^^^ 

 condenser circuit (see Fig. 6). If ^ p-Ll 



the former method is employed, 

 then an oscillation transformer is S| 

 used consisting of two coils of | 

 wire, one coil being inserted in the i- I — I 



condenser circuit and one in the 

 antenna circuit, and according as 

 these coils are near or far apart, they Fig. 6. 



are said to be closely or loosely 



coupled. These two circuits have then each their own natural 

 period of electric vibration, like tuning forks, and they have to be 

 adjusted to syntony. It is well known that under these conditions 

 oscillations set up in one circuit immediately create oscillations of 

 two frequencies in both circuits. This action can be easily illustrated 

 by two pendulums which are of the same length and are hung side 

 by side on a loose string distinguished by red and blue bobs. If 

 one pendulum is set swinging, it imparts little jerks to the other and 

 sets the latter in motion, but to do this the first must part with its 

 own energy, and hence is gradually brought to rest. Then the 

 operation is repeated in the reverse direction. The motion of each 

 pendulum may then be represented l)y the ordinates of a curve such 

 as those in Fig. 7. This kind of motion can, by a well-known 

 theorem, be resolved into the sum of two oscillations of different 

 frequencies. Hence, each pendulum may be said to possess two rates 

 of vibration. The same thing happens in the case of two closely 



