hertz's researches on electrical waves. 



147 



Fm. 1. 



In Fig. 1, A is an induction coil and B a discbarge. The wire con- 

 necting the knobs 1 and 2 of the si^arli micrometer 

 J/, consists of a rectangle, half a meter in length, 

 of copper wire 2 millimeters in diameter. This rect- 

 angle is connected with the secondary circuit of the 

 coil in the manner shown in the diagram ; and when 

 the coil is in action, sparks — sometimes several 

 millimeters in length — are seen to ])ass between the 

 knobs ] and 2, showing that there are violent elec- 

 trical oscillations, not only in the secondary circuit 

 itself, but in any conductor in contact with it. This 

 experiment shows even more clearly than the previ- 

 ous one that the rapidity of the oscillations is com- 

 parable with the velocity of transmission of electrical 

 disturbances through the copper wire, which, ac- 

 cording to all the evidence at our disposal, is nearly 

 equal to the velocity of light. 



In order to obtain micrometer sparks some millimeters in length, a 

 powerful induction coil is required, and the one used by Hertz was 52 

 centimeters in length and 20 centimeters in diameter, provided with a 

 mercury contact breaker, and excited by six large Bunseu cells. The 

 discharger terminals consisted of brass knobs 3 centimeters in diame- 

 ter. The experiments showed that the phenomenon depends to a very 

 great extent on the nature of the sparks at the discharger, the micro- 

 meter sparks being found to be much weaker when the discharge in 

 the secondary circuit took place between two points, or between a point 

 and a plate, than when knobs were used. The micrometer sparks were 

 also found to be greatly enfeebled when the secondary discharge took 

 place in a rarified gas, and also when the sparks in the secondary were 

 less than half a centimeter in length, while on the other hand, if they 

 exceeded li centimeters the sparks could no longer be observed be- 

 tween the micrometer knobs. The length of secondary spark which 

 was found to give the best results, and which was therefore employed 

 in the further observations, was about three-quaiters of a centimeter. 



Very slight diflerences in the nature of the secondary sparks were 

 found to have great effect on those at the micrometer, and Hertz states 

 that after some practice he was able to determine at once from the 

 sound and appearance of the secondary spark whether it was of a kind 

 to give the most powerful effects at the micrometer. The sparks which 

 gave the best results were of a brilliant white color, only slightly 

 jagged, and accompanied by a sharj) crack. 



The influence of the spark is readil;^ shown by increasing the distance 

 between the discharger knobs beyond the striking distance, when the 

 micrometer sparks disappear entirely, although the variations of po- 

 tential are now greater than before. The length of the micrometer cir- 

 cuit has naturally an important influence on the length of the spark, as 



