334 ELEMENTS OF ELECTRICITY AND MAGNETISM. 



pressure exerted upon it by the vibrating diaphragm, this causes 

 the battery current to fluctuate, the fluctuating battery current 

 induces an alternating current in the secondary 5 of the induction 

 coil and this alternating current passes over the line and actuates 

 the receiver telephone at the distant station. 



38. Wireless telegraphy. * The intensity of the magnetic 

 field in the neighborhood of an isolated magnet pole decreases 

 as the square of the distance increases, and the intensity of the 

 magnetic field at considerable distances from a complete magnet 

 (having two opposite poles) decreases as the cube of the distance 

 increases. The energy of a magnetic field is proportional to the 

 square of the field intensity and therefore the energy of the mag- 

 netic field in the neighborhood of an isolated pole decreases as 

 the fourth power of the distance increases, and the energy of the 

 magnetic field at considerable distances from a complete magnet 

 decreases as the sixth power of the distance increases. The 

 same laws of decrease of the energy apply in the case of the 

 electric field due to an isolated charge and in the case of the 

 electric field due to a doublet consisting of two opposite charges 

 near together, respectively. In the case of wave motion of any 

 kind which spreads out uniformly in all directions from a source, 

 the energy falls off as the square of the distance increases. 

 Therefore an enormously greater amount of energy can be 

 brought into action at great distances from a source of disturbance 

 by wave motion than by actions which produce a steady distribu- 

 tion of field. This remarkable property of wave motion is illus- 

 trated by the familiar fact that an audible effect may be produced 

 upon the ear of a distant person by the wave disturbance in the 

 air which is produced by a vibratory motion, whereas an ex- 

 tremely violent but steady circulation of air produced, for ex- 

 ample, by a powerful fan-blower, does not lead to any perceptible 

 energy manifestations at moderately great distances from the 

 blower. In consequence of the very great energy manifestations 



* This article describes the simple original arrangement which is due to Marconi. 



