Popular Science Monthly 



Li ; when it has built up to a fairly high 

 voltage negatively, the current in the 

 coil and gap circuit reverses and an in- 

 verse discharge begins in the opposite 

 direction. This also continues beyond 

 the zero voltage point, and results in a 

 positive charge of the condenser. Here 

 the condenser begins a third discharge, 

 this time in the same direction as at first. 

 Thus a rapidly reversing current is set 

 up in the condenser, coil and spark-gap 

 circuit, the successive swings of current 

 from one side to the other becoming 

 smaller and smaller until the energy is all 

 used up or withdrawn, or until the spark- 

 gap regains its normal non-conducting 

 condition and prevents further passage of 

 a spark. 



Detailed Study of Condenser 

 Voltage 



If we examine Fig. 36 a little more 

 closely we may see just what happens 

 throughout a full cycle of applied alter- 

 nating current (audio frequency) power. 

 Beginning at zero, the condenser voltage 

 builds up to about 9,500 in a little less 

 than one-half a thousandth of one second 

 and then, at the point A on the curve, 

 the high electrical pressure makes the 

 spark-gap conductive and the oscillatory 

 discharge begins. This discharge con- 

 sists of a number of rapid or radio fre- 

 quency alternations of potential (with 

 corresponding radio frequency alternating' 

 currents), and lasts for about one-quarter 

 of a thousandth of one second before the 

 energy is used up and the spark-gap again 

 becomes non-conductive. This occurs 

 at the point B of the curve. With the 

 spark-gap open (no spark passing) the 

 condenser begins to assume its normal 

 voltage from the audio frequency alter- 

 nating power applied to it, and rises to, 

 say, 5,000 volts at the point C. This 

 pressure is not enough to break down the 

 spark-gap, and consequently the conden- 

 ser potential follows the impressed po- 

 tential of the power transformer second- 

 ary, passing through zero at D (the end of 

 the first half cycle of power) and then 

 beginning to charge negatively or in the 

 reversed direction. At E the condenser 

 potential has reached 9,500 volts negative 

 i.e., with the lower plate positively 

 charged) and the spark-gap again becomes 

 conductive and allows the discharge to 



637 



pass through the primary oscillation cir- 

 cuit composed of the condenser, the 

 primary coil L, and the spark-gap. As 

 before, radio frequency oscillations con- 

 tinue for about quarter of a thousandth 

 of a second (to the point on the curve 

 marked CP) and then the gap becomes 



Fig. 36: How the radio frequency oscilla- 

 tions are produced by secondary discharge 



non-conductive. The normal charging 

 of the condenser follows through the high 

 point H and the zero point /, at the end 

 of the second half cycle or the first com- 

 plete cycle of applied power. Thereafter 

 the same series of operations is repeated, 

 and sparks representing a group of dwin- 

 dling radio frequency oscillations pass in 

 the middle of each half cycle. Thus, if 

 the applied power has a frequency of 500 

 cycles per second there will normally be 

 produced 1,000 sparks or groups of oscilla- 

 tions per second. 



Mechanical System for the Conversion 

 of Frequency 



We have evidently been considering 

 an arrangement of apparatus which will 

 convert, by way of the condenser dis- 

 charge, audio frequency alternating cur- 

 rent power into the radio frequency oscil- 

 lations which are necessary for wireless 

 signaling. The action may perhaps be 

 more vi\ddly appreciated if we consider a 

 similar mechanical system for increasing 

 frequency. Let us imagine a stiff spiral 

 spring S having a weight W hanging upon 

 it, and supported from a heavy beam B 

 as shown in Fig. 37. If a thin thread / 

 is tied to a hook set in the bottom of the 

 weight, we may slowly pull down on the 

 spring and weight system until the tension 

 on the spring is great enough to break the 

 thread. Then the weight will bob up- 

 ward rapidly, and its inertia will carry it 



