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 witli- 



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 



