High Frequency Apparatus Page 41

THE SPARK GAP 41 The number of studs or sparking points required will depend upon the diameter of the rotor disc and the speed at which it is to be driven. The number of studs and the speed govern the tone or pitch of the note imparted to the spark. With 12 points and a motor running at 1,800 R.P.M., the tone of the spark is musical and pleasant to the ear; this is in striking contrast to the crackling or crashing spark of the stationary gap. While this feature is of greater importance in the case of radio telegraph apparatus than with demonstration coils, still the pleasing musical note makes a good impression upon the audience. The distance between the rotating and the stationary electrode should, in general, be as short as possible without striking. The gap should certainly be adjustable by small degrees and the adjusting mechanism should preferably employ a screw with an insulated knob in order that the spark gap may be varied while the current is passing. The Quenched Gap.-By making the spark gap electrodes very massive, facing them off very accurately in a lathe, providing large radiation surface on each electrode, and, finally, by supporting the electrodes in such manner that the separation of their faces is but a few thousandths or perhaps hundredths of an inch, we have what is commonly termed the quenched gap. The large mass of metal 101 & Fig. 2.-Simple rotary gap