HERTZ S RESEARCHES ON ELECTRICAL WAVES. 



1G9 



l(Migth of wiiicli could be varied by means of a screw from a few liim- 

 dredths of a millimeter uj) to several millimeters. The dimensions 

 stated were sncli as to bring- the two conductors into unison, and. sec- 

 ondary sparks up to six or seven millimeters in length could be ob- 

 tained. 



Fig. 10. 



The circle was movable about an axis through its center perpendicu- 

 lar to its plane, to enable the position of the a.ir space to be varied. 

 Tiie axis was fixed in the position m 7i in the plane of A and A\ and 

 half way between them. The center of the circle was at a distance of 

 12 centimeters from the nearest points of A and A'. 



When / was in eitlier of the positions a or a' lying in the plane of A A' 

 no sparking occurred in the secondary, while maximum sparking took 

 place at b and // 00^ from the former positions. The e. m. f. giving rise 

 to the secondary sparks is, as in previous experiments, partly electro- 

 static and i)artly electro-magnetic, and the former being the greater will 

 determine the sign of the resultant e. m. f. The oscillations must, for 

 the reason previously explained, be considered as produced in the part 

 of the secondary most remote from the air space. Assuming the k. m. f. 

 and the amplitude of the resulting oscillation to be positive when / is 

 in the position i', they will both be negative when /is at b. 



When the circle was slightly lowered in its own plane the sparking 

 distance was increased at b' and diminished at b, and the null points 

 lay at a certain distance below a and a'. The electrostatic E. M. F. is 

 scarcely affected by such a displacement, but the integral of the e. m, f. 

 of induction taken round the circle is no longer zero, and therefore gives 

 rise to an oscillation which will be of i)ositive sign whatever be the po- 

 sition of/; for the direction of the resultant e. m. f. of induction is op- 



