Dr. 0. Lodge on Electric Radiation. 55 



L AI 



where — =21 log -j; I being the length of the entire rod 



portion of the oscillator, and d its diameter*. The measure- 

 ment of I is the most unsatisfactory part. It is best to 

 include the knobs and spark-gap as part of the whole length ; 

 the constriction at the spark will increase that part of the 

 self-induction, but the expanse of the knobs will diminish 

 another part. A trifle extra length should be allowed for 

 the currents in the disks or balls at the end ; but to measure 

 I from centre to centre is rather too much allowance. From 

 centre of one to nearest point of the other is a fair compromise. 

 As to S, it will be practically half f the static capacity of the 

 sphere or plate at either end of the oscillator, especially if 

 these are pretty big compared with the size of the rod. 

 Strictly speaking they are not isolated, even when far from 

 other conductors, because they are in presence of each other, 

 but the correction is usually small. For instance, for two 

 )ppositely charged spheres of radius r, at a considerable dis- 

 ance I from centre to centre, the capacity is about 



Hence the ordinary value of the capacity, as recorded for 

 convenience below, is always a minimum which circumstances 

 may increase but hardly diminish. 



S 

 Values of ~ for Isolated Bodies. 



For a globe, its radius. 



2 . 

 For a thin circular disk, - times its radius. 



For a thin square disk, 14& times inscribed circular disk, 



or *86 times a side of the square. 



For a thin oblong disk, a trifle greater than a square of the 



same area. 



Intensity of the Radiation. — Hertz has shown % that the 

 amount of energy lost per half swing, by a radiator of length 

 I charged with quantities -f Q and — Q at its ends respectively, 



18 7T 4 Q 2 / 2 



3K(i\) 3 ' 



* See Addendum at end. 



t Half, because the two spheres are technically "in series." See 

 Addendum at end of paper. 



X Wied. Ann. January 1889 ; or Nature, -vol. xxxix. p. 452. 



