240 Dr. 0. Lodge on the Electrostatic Force between 



more slowly, while at a very few wave-lengths (practically 

 one is sufficient) the first term under the root overpowers 

 the others, and the ordinary law of the inverse distance holds 

 good. 



To get an idea of the magnitude of this intensity at any con- 

 siderable distance from the axis, write Q = SV, where V is 

 measured by the length of spark employed at the oscillator, 



TT 



and write for q* its value y-^ ; then the amplitude of the 



electric force is 



F _ SV/./aK.sinfl 



Kp.LS ; 



/ 41 \ 

 or, as L = 21 fi (log -j — 1 J = 2 file, say, 



m=I™*° (9) 



Ire 

 Take as a numerical example any convenient oscillator, 

 say, to avoid unnecessary repetition of specification, the 

 small oscillator drawn to scale on page 54 of the Philoso- 

 sophical Magazine for July 1889, which emits waves 1 metre 

 long: let its constant c~4\, and its spark be, as there 

 quoted, 8 millimetres, so that V is about 26,000 volts. Then 

 the initial electric intensity at a distance of a couple of wave- 

 lengths in the equator is 



E _ 26000 volts 

 18 metres 

 = 14*4 volts per centimetre. 



Putting therefore at this distance of 2 metres a parallel wire 

 half a wave-length long as receiver, it utilizes 50 centimetres 

 of the above electromotive force, and gives a maximum spark- 

 ing potential of 720 volts, which corresponds to a spark-gap 

 of about a tenth of a millimetre between flat surfaces. This 

 is an upper estimate, because time for a quarter-period's dis- 

 sipation should be allowed, the result being multiplied by a 



dissipation-factor exp. ( |tr I I where R is to be found as 



follows. V v8U J 



Energy of Radiation. 

 The mean energy of the radiation per unit volume is, as is 



KE 2 



well known (Maxwell, art. 793),— — ; which in the present 



/ 41 \ 

 case, abbreviating the characteristic factor, ( log -z 1 J or its 



equivalent, to c, is ^ ' 



8tt- Vc 2 (W> 



