638 



Popular Science Monthly 



somewhat beyond the point of rest and 

 compress the spring. Thereafter the 

 weight will immediately start downward; 

 and it will continue to oscillate up and 

 down in shorter and shorter strokes until 

 the energy stored in the weight and spring 

 system has been used up. This cor- 

 responds in many ways to the circuit 



Fig. 37: Mechanical oscillating system 

 Fig. 38: Practical air-blast spark-gap 



shawn; pulling against the spring until 

 the thread breaks is comparable to charg- 

 ing the condenser until the spark-gap 

 breaks down, and the rapid up-and-down 

 oscillations of the weight and spring are 

 much like the rapid electrical oscillations 

 in the condenser and coil circuit. The 

 spring is analogous to the condenser C 

 and the weight to the inductance coil Lij 

 it is the stress of the spring (and in the 

 condenser) which trips off or breaks down 

 the restraining element (thread or spark- 

 gap), and it is the energy stored in the 

 weight (and in the inductance coil) which 

 carries the oscillations beyond dead center 

 on each swing and so keeps the system 

 vibrating. 



Controlling the Oscillation Frequency 



The two systems are alike as to another 

 important point, viz., the frequency of 

 the oscillations. We know from experi- 

 ence that the greater the mass of the 

 suspended weight and the greater the 

 flimsiness of the spring, the more slowly 

 the mechanical vibrations of the system. 

 By varying either or both of these we can 

 make the weight bob up and down at 

 almost any frequency we choose. In the 

 same way, the frequency of electrical 

 oscillations in the condenser and coil cir- 

 cuit is almost entirely dependent upon 

 the size of the condenser and coil. The 

 larger the condenser (the greater its 



capacity) and the larger the coil (the 

 greater its inductance), the slower the 

 radio frequency oscillations will be. Thus, 

 by altering the electrical constants of the 

 circuit (e.g., the capacity and inductance), 

 we can make the oscillation frequency 

 almost anything we desire. This matter 

 will be treated in greater detail later. 



The next point which should be con- 

 sidered here is the construction of a spark- 

 gap which will work regularly and con- 

 tinuously. Commercial radio practice 

 has brought out a great many types of 

 spark-gap, but years of experience have 

 shown that certain properties must be 

 secured if satisfactory operation is to be 

 expected. In the first place, the gap must 

 always break down at some definite volt- 

 age. It is evident from Fig. 36 that if 

 the potential which established conduc- 

 tivity across the gap varied, the oscilla- 

 tions would begin at different points in 

 each half-cycle and that the oscillation 

 groups would not occur regularly. If 

 the break-down potential were normally 

 9,500 but sometimes became 8,000, when 

 the lower value held the oscillations would 

 start off too soon in the half-cycle, and 

 the full discharge of the condenser would 

 not be utilized. If it ran up to 11,000 

 volts, no spark would pass at all, and the 

 charge for that particular half-cycle 

 would be practically wasted in so far as 

 the production of a group of radio fre- 

 quency oscillations was concerned. Uni- 

 formity of sparking potential depends 

 upon keeping the gap cool more than on 

 anything else, since the hotter the gap 

 the lower the potential at which it breaks 

 down. For small powers the necessary 

 cooling may be secured by making the 

 spark-gap terminals large, since then the 

 heat will be carried away rapidly by the 

 mass of metal. For larger powers some 

 form of artificial cooling is used. 



A Successful Cooled Spark-Gap 



A form of air-cooled gap which has 

 been found satisfactory for many pur- 

 poses, is shown in Fig. 38, and which is 

 largely used by the French. It consists 

 merely of a brass or copper tube forming 

 one electrode and placed endwise to a 

 Hat plate which acts as the other terminal. 

 A blast of air is fed through the tube by 

 way of a rubber hose, and spreads out 



