hertz's researches on electrical waves. 187 



The air space was tlieii placed at such a point and close to tlie wall 

 and was theu moved slowly away from the wall, wheu it was found that 

 while there was no sparkiug quite close to the metal plate, it began at 

 a very small distance from it, rapidly increased, reached a maximum at 

 the point B, and then diminished again. At C the sparking again be- 

 came excessively feeble, and increased as the circle was moved still 

 further away. The sparking continued steadily to increase after this, 

 as the motion of the circle was continued in the same direction, owing, 

 as before, to the direct action of the i)rimary oscillation. 



The curves shown by the continuous lines in Fig. 12 were obtained 

 from the results of these experiments, the ordinates representing the 

 intensity of the sparks at the distances represented by the correspond- 

 ing abscissas 



The existence in the electrical waves of nodes at A and C, and of 

 loops at B atid D, is fully established by the experiments which have 

 been described; but in another sense the j^oints P> and D may be re- 

 garded as nodes, for they are the nodal points of a stationary wave of 

 magnetic induction which, according to theory, accompanies the elec- 

 trical wave and lags a quarter wavelength behind it. 



This can easily be shown to follow from the experiments, for when the 

 secondary circle is ]>laced in the plane of vibration with the air space 

 at its highest point, there will be no sparking if the E. M. F. is uniform 

 throughout the space occupied by the secondary. This can only take 

 place if the e, m. f. varies from point to point of the circle, and if its 

 integal round the circle differs from zero. This integral is proportional 

 to the number of magnetic lines of force passing backwards and for- 

 wards across the circle, and the intensity of the sparks may be consid- 

 ered as giving a measure of the magnetic induction, which is perpen- 

 dicular to the plane of the circle. Now in this position vigorous 

 sparking was observed close to the wall, diminishing rapidly to zero 

 as the point B was approached, then increasing to a maximum at C, 

 falling to a well-marked minimum at D, and finally increasing continu- 

 ously as the secondary approached still nearer to the primary. If the 

 intensities of these sparks are taken as ordinates, positive and negative, 

 and the distances from the wall as abscissie, the curve shown by the 

 dotted lines in Fig. 12 is obtained, which therefore represents the mag- 

 netic waves. 



The phenomena observed in the tirst series of experiments described 

 in this pai)er may therefore be regarded as due to the resultant electric 

 and magnetic actions. The former changes sign at A and C, the latter 

 at B and D, so that at each of these points one part of the action changes 

 sign, while the other does not, and therefore the resultant action, which 

 is their product must change sign at each of these points, as was found 

 to be the case. 



When the secondary circle was in the plane of vibration the sparking 



