hertz's researches on electrical waves. 149 



cuit just as au elastic bar is thrown into detiuite vibrations by blows 

 from a hamuier. If this assumtion is correct, the condition for the dis- 

 appearance of the sparks at M will be that the vibration periods of the 

 two branches e 1 and e2 shall be equal. These periods are determined 

 by the products of the coefficients of self-induction of these conductors 

 into the capacity of their terminals, and are practically independent of 

 their resistances. 



In confirmation of this, it is found that if when the point of contact is 

 at e and the sparks have been made to re-appear by connecting- a con- 

 ductor with one of the knobs, this conductor is replaced by one of 

 greater capacity, sparking is greatly increased. If a conductor of equal 

 capacity is connected with the other micrometer knob the sparks disap- 

 pear again ; the effect of the first conductor can also be counteracted 

 by shifting- the i)oiut of contact towards it, thereby diminishing the 

 self-induction in that branch. The conclusions were further confirmed 

 by the results obtained when coils of copper wire were inserted into one 

 or other and then into both of the branches of the micrometer circuit. 



Hertz supposed that as the self-induction of iron wires is, for slow 

 alternations, from eight to ten times that of copper wires, therefore a 

 short iron wire would balance a long copper one; but this was not 

 found to be the case, and he concludes that, owing to the great rapidity 

 of the alternations, the magnetism of the iron is unable to follow them 

 and therefore has no effect on the self-induction.* 



Induction phenomena in open circuits. — In order to test more fully his 

 conclusion that the sparks obtained in theex[)eriments described in the 

 previous section, .were due to self-induction, Dr. Hertz placed arectangle 

 of copper wire with sides 10 and 20 centimeters in length, respectively, 

 broken by a short air si)ace, with one of its sides parallel and close to 

 various portions of the secondary circuit of the coil, and of the micro- 

 meter circuit, with solid di-eleetrics interposed, to obvijite the possibil- 

 ity of sparking across, and he found that sparking in this rectangle 

 invariably accompanied the discharges of the induction coil, the longest 

 S])arks being obtained when a side of the rectangle was close to the dis- 

 charger. 



A copper wire, i g h (Fig. 3), was next attached to the discharger, 

 and aside of the micrometer circuit, which was sup[)orted on an insu- 

 lating stand, was placed parallel to a portion of this wire, as shown in 

 the diagram. The sparks at M were then found to be extremely feeble 



* lu a note in Wiedemann's Annalen, vol. xxxi, page 543, Dr. Hertz statest hat since 

 the publication of his paper in the same volume, he had found that Von Bezold had 

 published a paper in 1870 (PoggcudorJf's Annalen, vol. CXL, page 541), in which he 

 had arrived by a different method of experiuionting at similar results and conclusions 

 as those given by him under the head of Preliminary Experiments. 



