Prof. Hertz on Rays of Electric Force. 291 



long and 1 millim. thick, led, parallel to each other and at 

 right angles to the first-mentioned wires, to a spark-interval 

 similarly arranged to that of the circular conductor. In this 

 conductor no attention was paid to resonance, which is here 

 not verj perceptible. It would have been more simple to 

 place the spark-interval directly in the middle of the straight 

 wire ; but the spark-interval could not then have been mani- 

 pulated and observed in the focus of the concave mirror 

 without the observer obstructing the aperture of the mirror. 

 For this reason the arrangement described was preferred as 

 more advantageous. 



The Production of the Ray. 



If the primary vibration be now set up in a large free 

 space, the circular conductor in its neighbourhood permits 

 the observation, on a smaller scale, of all the phenomena 

 which I had previously observed and described in the neigh- 

 bourhood of a larger oscillation*. 



The greatest distance at which sparks could be recognized 

 in the secondary conductor was 1'5 metre, or as much as 

 2 metres with a favourable condition of the primary spark- 

 interval. 



The effect is increased on either side if a plane conducting 

 surface is adjusted on the opposite side, parallel to the oscilla- 

 tion and at a suitable distance. If the distance is chosen 

 either very small or somewhat greater than 30 centim. the 

 surface produces a prejudicial ejffect ; it produces a strong 

 reinforcement at distances between 8 and 15 centim,, a feeble 

 reinforcement at 45 centim., and is without effect at greater 

 distances. We have previously remarked upon this pheno- 

 menon, and we conclude from it that the wave corresponding 

 to the primary oscillation has, in air, a half wave-length of 

 about 30 centim. We might expect a still greater reinforce- 

 ment by replacing the plane surface by a concave mirror of 

 the form of a parabolic cylinder, in the focus of which the 

 longer axis of the primary oscillation falls. If the mirror is 

 properly to concentrate the action at a distance, it is advan- 

 tageous to have its focal length as small as possible. But if 

 the direct w^ave is not immediately to quench the action of the 

 reflected wave, the focal length must not be much less than a 

 quarter of a wave-length. 1 chose therefore a focal length of 

 12^ centim., and constructed the mirror by bending a zinc 

 plate, 2 metres long, 2 metres broad, and -^ millim. thick, 

 round a wooden frame of the rioht curvature into the desired 



* H. Hertz^ Wiedemann's A7makn, xxxiv. pp. 155, 551, 009. 



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