332 ' BELL SYSTEM TECHNICAL JOURNAL 



connected with electric oscillations; an interest extending back to 

 1879, when, at the suggestion of Helmholtz, he had considered tackling 

 a prize problem proposed by the Berlin Academy of Science aimed at 

 the proof of a portion of Maxwell's theory, but which he had abandoned 

 for the reason that oscillations of sufficiently high frequency were not 

 then available. While using in his lectures at Karlsruhe a pair of 

 flat ("pancake") coils, called Reiss or Knockenhauer spirals, mounted 

 adjacent to each other, he was surprised to find how easy it was to 

 obtain sparks between the terminals of the secondary coil when a small 

 Leyden jar or even a small induction coil was discharged through the 

 primary, provided the primary discharge took place across a spark 

 gap. This, of course, was an indication of an exceptionally strong in- 

 ductive effect. This observation led to his discovery of a method of 

 exciting electric disturbances far more rapid than any hitherto known, 

 such as those of Leyden jars or open induction coils as customarily 

 used, and having wave-lengths, it turned out, capable of being meas- 

 ured within the confines of a laboratory. His oscillator was nothing 

 more than a short metal rod with a spark gap in the middle (sometimes 

 with metal spheres or plates attached to the ends, resembling a dumb- 

 bell), the sparking terminals consisting of small knobs or spheres which 

 were connected to the terminals of a Ruhmkorff induction coil; the 

 small inductance and capacitance of this simple linear conductor, to- 

 gether with the proper functioning of the spark gap, accounting for its 

 success. By such means Hertz obtained wave-lengths from a few 

 meters down to 30 centimeters, and so began, it is seen, with the " ultra- 

 short" waves that are again coming into vogue. 



Hertz began his experiments with a study of the "induction " about 

 this exciter. As he commented in his first paper in this series, "On 

 Very Rapid Oscillations," published in May, 1887, theory had pre- 

 dicted the possibility of very rapid oscillations in open-wire circuits of 

 small capacitance, but it could not be predicted from theory whether 

 they could be produced on such a scale as to admit of their being ob- 

 served. Hertz not only devised a method of producing such oscilla- 

 tions, but also discovered a method of detecting them, by their effects 

 in the surrounding space. His detector consisted merely of a short 

 length of wire bent in the form of a rectangle or a circle and containing 

 a micrometer spark gap, across which minute sparks could be seen in a 

 darkened room; especially if this secondary circuit was in electrical 

 resonance with the exciter. This exceedingly simple detector was 

 indeed a capital discovery. Some five years earlier Professor G. F. 

 Fitzgerald, of Dublin, had suggested "the combination of a vibrating 

 generating circuit with a resonant receiving circuit ... as one by 



