ii6 POPULAR SCIENCE MONTHLY. 



harmonics of the fundamental one; that is to say, notes which have 

 frequencies represented by the numbers 2, 3, 4, 5, etc. If, however, 

 the pipe is closed at the top, then over-blowing the pipe makes it yield 

 the odd harmonics or the tones which are related to the primary tone 

 in the ratio of 3, 5, 7, etc., to unity. Accordingly, if a stopped pipe 

 gives as its fundamental the note C, its first overtone will be the fifth 

 above the octave or Gr'. 



As already remarked, the aerial wire or radiator as used in Mar- 

 coni telegraphy may be looked upon as a kind of ether organ-pipe or 

 siren tube, and its electrical phenomena are in every respect similar 

 to the acoustic phenomena of the ordinary closed organ-pipe. When 

 the aerial is sounding its fundamental ether note, the conditions which 

 pertain are that there is a current flowing into the aerial at the lower 

 end, but at that point the variation in potential is very small, whereas 

 at the upper end there is no current but the variations of potential are 

 very large. Accordingly, we say that at the upper end of the aerial 

 there is an antinode of potential and a node of current, and at the 

 bottom, an antinode of current and a node of potential. By altering 

 the frequency of the electrical impulses we can create in the aerial 

 an arrangement of nodes of current or potential corresponding to the 

 overtones of a closed organ-pipe. But whatever may be the arrange- 

 ment, the conditions must always hold, that there is a node of current 

 at the upper end and an antinode of current at the lower end. In 

 other words, there are large variations of current at the place where 

 the aerial terminates on the spark gap and no current at the upper 

 end. The first harmonic is formed where there is a node of potential 

 at one third of the length of the aerial from the top. In this case, 

 we have a node of potential not only at the lower end of the wire, but 

 at two thirds of the way up. In the same way we can create in the 

 closed organ-pipe by properly overblowing the pipe, a region about two 

 thirds of the way up the pipe, where the pressure changes in the air 

 are practically no greater than they are at the mouthpiece. We can 

 make evident visually in a beautiful manner the existence of similar 

 stationary electrical waves in an aerial by means of an ingenious 

 arrangement devised by Dr. Georg Seibt, of Berlin. It consists of 

 a very long, silk covered copper wire A (see Fig. 10) wound in a close 

 spiral of single layer round a wooden rod six feet long and about two 

 inches in diameter. This rod is insulated, and at the lower end the 

 wire is connected to a Leyden jar circuit, consisting of a Leyden jar 

 or jars and an inductance coil L, the inductance of which can be 

 varied. Oscillations are set up in this jar circuit by means of an 

 induction coil discharge, and the lower end of the long spiral wire is 

 attached to one point on the jar circuit. In this manner we can com- 

 municate to the bottom end of the long spiral wire a scries of electric 



