192 hertz's researches on electrical waves. 



raiigemeiit used is (speaking roughly) about the 30- rdV, -0-5-0 (one/ thirty- 

 millionth) of a second. 



If there be conductors in the neighborhood of this " vibrator," cur- 

 rents will as usual be induced in each on every passage of the charge 

 between the plates, each passage serving simply as a primary current. 



Now, speaking briefly, the whole object of the experiment is to lind 

 out if these induced currents take place simultaneously in conductors 

 situated at various distances from the primary current, and if not, to 

 determine the delay. In order to do this we must, in the first place, be 

 possessed of some means of even ascertaining that these currents occur, 

 all ordinary methods being inadequate for detecting currents lasting 

 only for such exceedingly short periods as these do. By devising how 

 to determine the existence of these currents, Hertz made the experi- 

 ment possible. 



His method depends on the principle of resonance, previously sug- 

 gested by Fitzgerald, and his current-observing apparatus is simply a 

 conductor, generally a wire bent into an unclosed circle, which is of such 

 a length that if a current be induced in it by a passage of a charge 

 across the " vibrator " the return current or rush back of the electricity 

 thus T)roduced in the ends of the wire occurs simultaneously with the 

 next impulse, due to the passage back across the " vibrator." 



In this way the current in the " resonator " increases every time, so 

 that at last the end charges, which are always of opposite sign, grow 

 to be so great that sparks will actually occur if the ends of the wire are 

 brought near together. Thus Hertz surmounted the diflflculty previously 

 experienced by Fitzgerald when proposing electro-magnetic interference 

 experiments. 



The time of vibration in this circle is, as before, In V(CI), but on account 

 of difficulties in calculating these quantities themselves, the length of the 

 wire is most readily found by trial. To suit the " vibrator" we used, it 

 was about 210 centimeters of wire No. 17. The ends of the wire were 

 furnished with small brass knobs, which could be adjusted as to dis- 

 tance between them, by a screw arrangement, the whole being mounted 

 on a cross of wood for convenience in carrying about. 



At first sight the simplest "resonator" to adopt would seem to be 

 two more plates arranged similarly to the " vibrator," but it will be seen 

 on consideration that it would not do, because no break for seeing the 

 sparking could be put between the plates, for if it were, the first in- 

 duced current would be too feeble to jump the break, so that the re- 

 enforcement stage could never begin.* 



The charging of the "vibrator" was efl'ected by connecting the ter- 

 minals of an induction-coil with the plates. In this way a continuous 

 shower of sparks could be obtained in the resonating circle. 



* However, two ))air8 arranged in line, the pairs connected by a wire, could prob- 

 ably be fjot to spark between the center platcH. 



