364 POPULAR SCIENCE MONTHLY. 



same balls in air at normal pressure. This shows that there is a very 

 great advantage in taking the discharge spark in compressed air. A 

 better effect can be produced by substituting dry gaseous hydrochloric 

 acid for air at ordinary pressures. 



One other incidental advantage is that the noise of the spark is very 

 much reduced. The continual crackle of the discharge spark of the 

 induction coil in connection with wireless telegraphy is very annoying 

 to sensitive ears, but in this manner we can render it perfectly silent. 

 Professor Fessenden also states that when the spark balls are sur- 

 rounded by compressed air, and if one of the balls is connected with 

 a radiator, the compression of the air, although it shortens the spark 

 gap corresponding to a given voltage, does not in any way increase the 

 radiation. When, however, the air in the spark ball vessel is com- 

 pressed to 60 lbs. in the square inch, there is a marked increase in 

 the effective radiation, and at 80 lbs. per square inch the energy emitted 

 in the form of waves is nearly three and a half times greater than 

 at 50 lbs., the potential dift'erence between the balls remaining the 

 same. 



This effect is no doubt connected with the fact that the produc- 

 tion of a wave, whether in ether or in any other material, is not so 

 much dependent upon the absolute force applied as upon the sudden- 

 ness of its application. To translate it into the language of the 

 electronic theory, we may say that the electron radiates only whilst 

 it is being accelerated, and that its radiating power, therefore, depends 

 not so much upon its motion as upon the rate at which its motion is 

 changing. 



The advantage in using compressed air round the spark gap is that 

 we can increase the effective potential difference between the balls 

 without rendering the spark non-oscillatory. In air of the ordinary 

 pressure there is a certain well-defined limit of spark length for each 

 voltage, beyond which the discharge becomes non-oscillatory, but by 

 the employment of spark balls in compressed air, we can increase the 

 potential difference between the balls corresponding to a given dis- 

 tance apart before a discharge takes place, or employ higher poten- 

 tials with the same length of spark gap. In addition to this, we have, 

 perhaps, the production of a more effective radiation, as asserted by 

 Fessenden, when the air pressure exceeds a certain critical value. 



The next element which we have to consider in the transmitting 

 arrangements is a condenser of some kind for storing the energy which 

 is radiated at intervals. Where a condenser other than the aerial is 

 employed for storing the electric energy which is to be radiated by 

 the aerial, some form of it must be constructed which will withstand 

 high potentials. As the dielectric for such a condenser, only two 

 materials seem to be of any practical use, viz., glass and micanite. 



