Page:Popular Science Monthly Volume 92.djvu/652

 G36

��Popular Science Monthly

��at any point corresponding to any par- ticular instant. The vertical or voltage axis on the left is marked off to show positive voltages above the central or zero point, and negative voltages below. If we follow along the curve we find that at the beginning, at 1/lODO of a second, at

���Fig. 35 : Graphical representage of the five hundred cycle secondary voltage

2/1000 second, at 3/1000 second (and consequently at each one-thousandth second or at the end of each half cycle) the voltage of the condenser is zero. This is shown by the fact that the wavy line crosses the zero line at the point cor- responding to each of these instants, and means that for the moment the condenser has no charge. If we look for the points of maximum voltage, we find that at half a thousandth of one second the condenser is charged to 10,000 volts positive, at one and one-half thousandths to 10,000 volts negative, at two and one-half thou- sandths to 10,000 volts positive again and so on continuously. In the same way we find that, starting from zero time and zero voltage (at the extreme left of the diagram) the voltage gradually rises in the positive direction, reaching about 7000 volts in one quarter of a thousandth of one second, passing through the high point just mentioned, and then falls to zero and begins a similar operation in the reversed direction.

The Charge in the Condenser

The condition just examined is based upon the assumption that nothing is con- nected to the wires X and Y. When the condenser is charged either positively or negatively, a certain definite amount of electrical energy is stored in it for the time being. This energy may be allowed to flow back into the secondary coil S, as has just been shown, or it may be with-

��drawn from the condenser for some more useful purpose. The amount of electrical energy thus stored may have large values for a time; the quantity depends entirely upon the capacity of the condenser (its storing ability) and the voltage to which it is charged. Obviously, to take the energy out usefully one must have some method of catching the condenser when it is charged; to get the ir.ost energy from each half cycle, the charge must be with- drawn at the instant of maximum voltage. This requires some automatic device which works regularly and quii^kly, since the highest voltage occurs only each one- thousandth of a second and lasts for only a few ten-thousandths of one second.

How the Condenser Discharges Through the Spark -Gap

Let us suppose that the condenser is shunted by the circuit of Fig. 32, which shows the spark-gap <S connected across it through the primary Li. If the spark- gap consists of two electrodes which are separated widely, there will be no new effect; if, on the other hand, the spark- gap points are brought within about }^2 i^i- of each other, the potential of 10,000 volts will be sufficient to break across the air space. This will cause an entirely new and useful sequence of events, as may be seen from the following: If the gap electrodes are separated to exactly the distance. which permits a spark to pass when a voltage of 9,500 is applied across them, it is evident that it will not be possible to charge the condenser to 10,000 volts pressure. This is because when the voltage has risen to the breakdown point of 9,500 volts, the energy in the condenser will discharge across the gap in the form of an electric spark. The illustration Fig. 36 will serve to give a rough idea of how the condenser potential is affected; following the voltage line from zero at the left, it is seen that when the potential of 9,500 is reached there is a sudden drop through zero voltage and on farther down to about 8,000 volts negative. This hap- pens because ajl the stored electrical energy rushes across the spark-gap S through the inductance (primary coil) Li shown in Fig. 32. The discharge does not stop at zero voltage, but continues farther in the .same direction because of the magnetic effect of the primary coil

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