HERTZ S EKSKAKCHES ON ELECTRICAL WAVES. 



163 



When the air space was at its greatest distance from the plane the 

 sparking distance attained a maximum value of from 2 to 3 millimeters. 

 The sparks were shown tobe due to the fundamental vibration, by slightly 

 varying the secondary, so as to throw it out of unison with the primary, 

 when the sparking distance was diminished, which would not have been 

 the case if the sparks had been due to overtones. Moreover, the sparks 

 disappeared when the secondary was cut at its points of intersection 

 Avitli the horizontal plane through the primary, though these would be 

 nodal points for the first overtone. 



When the air space was ii;ept at its greatest possible distance from 

 the horizontal plane through the primary, and turned about a vertical 

 axis, the sparking distance attained two maxima at the points for which 

 ^=0, and almost disappeared at the points for which ^=90'^. 



Fig. 8. 



The lower half of Fig. S .shows the different positions of minimum 

 sparking, A A' is the primary conductor, and the lines m n re])resent 

 the projections of the secondary circuit on the horizontal plane. The 

 arrows perpendicular to these give the direction of the resultant lines 

 of force. As this did not anywhere vanish in passing from the sphere 

 A to the sphere A', it could not change its sign. 



The diagram brings out the two following points : 



(1) The distribution of the resultant E. m. f. in the vicinity of the 

 rectilinear vibration is very similar to that of the electro-static E. M. f. 

 due to the action of its two extremities. It should be specially noted 

 that near the center of the primary the direction is that of the electro- 

 static E. M. F., showing that it is more powerful than the electro- dynamic, 

 as required by theory. 



(2) The lines of force deviate more rapidly from the line A A' than 

 the electro-static lines, though this is not soevident on the reduced scale 

 of the diagram as in the author's original drawhigs on a much larger 

 scale. 



It is due to the components of the electro-static E. M. F. parallel to 

 A A' being weakened by the E. m. f. of induction, while the perpendic- 

 ular components remained unaffected. 



