Page:Popular Science Monthly Volume 92.djvu/653

 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successive swings of current from one side to the other becoming smaller and smaller until the energy is all used up or withdrawn, or until the spark- gap regains its normal non-conducting condition and prevents further passage of a spark.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

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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

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