Popular Science Monthly 



937 



the condenser one at a time, or small 

 additional currents must be introduced 

 by way of the inducti\'e portions of the 

 circuit. These charges must be applied 

 at the instant that the natural (or self- 

 oscillating) charge of the condenser is 

 of their polarity, for otherwise no 

 advantage of increased charge would be 

 had ; similarly, the increments (or 

 additions) of current must be made 

 when the natural current is flowing in 

 the proper direction, for, if not, there 

 would be an opposition to the normal 

 current in the circuit and no increase 

 would be secured. This is as certain as 

 the fact that, in order to make a swing 

 go higher and higher, it must be pushed 

 when it is moving or about to move in the 

 same direction as the applied force; and 

 it is true for the same reason. 



Let us now assume that the alternator 

 E in Fig. i is capable of delivering 

 I kilowatt of electrical power at 50,000 

 cycles per second, but can run safely at 

 speeds as high as that giving 100,000 

 cycles. Let the inductance and capacity 

 be of such values that the natural 

 frequency of the circuit is 50,000 cycles 

 per second (corresponding to a wave- 

 length of 6,000 meters), and consider 

 that the total resistance is two ohms. 

 If the alternator is started from rest 

 and gradually speeded up, it will 



Fig. 6. A closed and an open circuit (A and B) 

 are used to radiate waves to a receiving 

 antenna, C 



produce pulses of alternating voltage 

 at gradually increasing frequencies. 

 These voltage impulses will charge the 

 condenser C first in one direction and 

 then in the other; but very little current 

 will flow, because there is no tendency 

 for these lower-frequency voltages to 

 co-operate by resonance. As the 

 frequency comes close to 50,000 per 

 second, however, the current will corn- 



Fig. 5. Simple 

 antenna circuit 



mence to rise, and at 50,000 cycles it 

 will reach a maximum of about 23 

 amperes. At this frequency the small 

 voltage additions produced by each 

 cycle of the alternator are impressed 

 upon the condenser exactly in step with 

 the natural oscillation voltages, and the 

 greatest possible oscillation current 

 results. When the frequency is increased 

 beyond 50,000 cy- 

 cles, the resonant 

 value of the circuit, 

 the circuit begins 

 to fall off very rap- 

 idly. If one meas- 

 ures the current at __^ 

 each of a set of fre- 

 quencies near the tuned point, the result 

 may be plotted in the form of a curve like 

 that of Fig. 3, where the intersection 

 over each frequency shows the amount 

 of current indicated by / when the 

 alternator is run at the corresponding 

 speed. It should be noted that the rise 

 and fall are extremely sudden. 



Suppose now that this same experi- 

 ment be repeated with all conditions 

 remaining the same, except that the 

 total resistance of the circuit is set at 

 10 ohms. As the speed of the alternator 

 is increased it is noted that the current 

 begins to rise in the neighborhood of 

 50,000 cycles, as before, and to fall 

 after that speed is passed; the interest- 

 ing features are, however, that the 

 maximum current is now only 10 

 amperes, and that the rise and fall 

 near the resonant point are not nearly 

 so sudden as before. By taking a 

 series of careful measurements and 

 plotting them out in curve form, a 

 diagram like that of Fig. 4 may be 

 produced. The slope of the sides of 

 this curve is considerably less than that 

 of Fig. 3 ; the effects of adding resistance 

 have evidently been to decrease the 

 current at resonance, and to make the 

 circuit less sharply dependent upon 

 applied frequency. \Vc know that this 

 means the tuning of the circuit has 

 become less sharp; we know also, that 

 the adding of resistance has increased 

 the damping of the free oscillations in 

 the circuit. These two results are 

 closely related. 



Next, the application of these experi- 

 ments to a modern radio telegraph 



