Page:Popular Science Monthly Volume 88.djvu/964

 936

��Popular Science Monthly

��oscillation may be determined by wave- meter measurement, or may be computed according to the rule given in the March article. Speaking generally, what hap- pens is that the dielectric of the

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Fig. 3. Curve showing the rise and fall of current by varying the frequency

condenser is electrically strained in one direction when the charging voltage (or pressure) is applied to it ; as soon as the pressure is relieved, the strain reacts and its energy produces a current through the circuit (Fig. i) from the positive plate of the condenser toward the negative side. In passing through the inductance the current sets up a strong magnetic field, which expands and stretches away from the coil as the current through it grows larger. Since there was only a definite amount of electrical energy forced into the conden- ser by the original charging voltage, there is a limit to the amount of current which can be produced by the discharge; as soon as this limit is reached the magnetic field around the coil L begins to contract, and adds its energy to the current flowing toward the negative side of the condenser. By this time the condenser is fully discharged, that is, the two plates are at the same potential. But the magnetic field is still collapsing on the coil, and therefore, current is forced to continue flowing in the same direction as before; this results in a piling up of potentials on the "negative" plate of the condenser and a reduction of electrical pressure on the plate which was "positive." In other words, the reaction of the magnetic field has

��caused the condenser to assume a new charge, of polarity opposite to that which it had originally. The pressure of this inverted charge increases until the energy of the magnetic field is exhausted; then the condenser dis- charges once more, but in the opposite direction. A current flows back through the inductance, and an expanding field is set up, just as before, except that the polarity is reversed. The contraction of this second magnetic field forces a new charge upon the condenser, and this time the polarity is the same as of that which began the oscillation. Since a limited amount of energy is set free in the circuit, and since some of this energy is used in heating the wires (because of their resistance) each suc- cessive charge and each successive current is smaller than that which preceded it, and the free oscillation is damped, as shown in Fig. 2. The greater the resistance of the circuit the greater the proportion of the original energy, which is lost in heat at each oscillation, and the sooner the current is damped down to a very small value.

What has this internal action of a resonating circuit got to do with its resonant condition, or its tuning (which is much the same thing)? In a word, everything. Why? Because "tuning"

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��4. Curve showing changes in current by altering the resistance

��is little more than taking advantage of this self-swinging power of a circuit so that energy may be added to it at just the right time to give its oscillations the largest amplitudes possible. In adding small amounts of energy to an oscillating electrical system, the addition must be made by the application to it of corre- sponding magnetic or electric forces. That is, small charges must be put upon

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