hertz's researches on electrical waves. 175 



round the circle, vigorous sparking was observed in .ill positions. The 

 sparking distance attained its maximum length of about six millimeters 

 when its air space was nearest to that of the primary, and its minimum 

 length of about three millimeters when the distance between the two 

 air spaces was greatest. If the secondary had been influenced by the 

 electrostatic force, sparking would only be expected when the air space 

 was close to the base-line, and a cessation of sparks in the intermediate 

 positions. The direction of the oscillation would, moreover, be deter- 

 mined by the direction of the e. m. f. in the portion of the secondary 

 furthest from the air space. There is however superposed upon the 

 electro-statically-excited oscillation a second oscillation due to the e. 

 M. F. of induction, which produces a considerable effect since its inte- 

 gral round the circle (considered as a closed circuit) does not vanish ; 

 and the direction of this integral e. m. f. is independent of the position 

 of the air space, opposing the electro-static E. M. F. in the portion of 

 the secondary next to A A', and assisting it in the portion furthest from 

 A A', as explained in a previous paper. 



The electro-static and electro dynamic E. m. f.s therefore act in the 

 same direction when the air space is turned towards the primary con- 

 ductors, and in opposite direction when the air space is turned away 

 from the primary. In the latter position it is tlie E. M. F. of induction 

 which is the more powerful, as is shown by the fact that there is no dis- 

 appearance of sparking in any i)osition of the air space, for when this 

 is 90 degrees to the right or left of the base line it coincides with a node 

 with respect to the electrostatic E. M. F. In these positions the in- 

 ductive action in the neighborhood of the primary can be observed, in- 

 dependently of the electrostatic action. 



Waves in Rectilinear Wires. — In order to produce in a wire by means 

 of the primary oscillations a series of advancing waves of the character 

 required for these experiments, the following arrangements were 

 made : Behind the plate A was placed a plate P of equal size. A 

 copper wire one millimeter in diameter connected P to the point m of 

 the base-line. From m the wire was continued in a curve about a meter 

 in length to the point n, situated about 30 centimeters above the air 

 space, and was then further continued ia a straight line parallel to the 

 base-line for such a distance as to obviate all danger of disturbance from 

 reflected waves. In the present series of experiments the wire passed 

 through a window, and after being carried to a distance of about 60 

 meters was put to earth, and a si>ecial series of experiments showed 

 that this length was sufficient. When a wire, bent so as to form a 

 nearly closed circuit with a small air space, was brought near to this 

 straight wire, a series of fine sparks was seen to accompany the dis- 

 charges of the induction coil. Their intensity could be varied by vary- 

 ing the distance between the plates P and A. The wa\'es in the recti- 

 linear wire were of the same period as tlint of the primary os(;il]ations, as 

 was proved by their being shown to be m unison with each of the two 



