328 PROCEEDINGS OF THE AMERICAN ACADEMY. 



free end by an amount a little less than half the distance apart of the 

 wires. This correction, applied to my experiments, amounts to less than 

 one per cent in the case even of the shortest parallel- wire oscillator used 

 in the calibration, and has been taken into account. 



That the velocity of the waves on the wires is equal to the velocity 

 of light has its theoretical basis in the fact that for rapid oscillations 

 guided by parallel wires, the self-induction per unit of length multiplied 

 by the capacity per unit of length is the reciprocal of the square of the 

 velocity of light. That the velocity of propagation on the parallel 

 wires is the velocity of light has been shown experimentally by Trow- 

 bridge and Duane 4 and by Saunders. 5 Recently also Diesselhorst 6 of 

 the lleichsanstalt has made some experiments which indicate that the 

 wave-length on the parallel wires differs from the wave-length in air 

 by less than one-third of one per cent when the parallel wires are not 

 more than 100 meters long. 



\V<ii* -length of the Warn Produced by the Hertz Oscillator. — If now 

 we take the two parallel wires, separate them, and extend them out 

 oppositely so as to form a Hertz oscillator, the capacity per unit of 

 length diminishes, while the inductance per unit of length increases. 

 Does the wave-length remain the same ; namely, four times the length 

 of the half-oscillator, or a = 2 /, where / is the length of the whole 

 oscillator 1 Some theoretical writers (Abraham, 7 Rayleigh 8 ) say that 

 it does remain very approximately the same (if the diameter of the wire 

 is a small fraction of the length) ; while, on the other hand, Macdonald 9 

 has concluded that a is equal to 2.53 I, and he is supported in this con- 

 clusion by Pollock and Close. 10 



Experimental tests of the question have heretofore usually been 

 made with very short vibrating systems, to which the theoretical de- 

 ductions are not directly applicable. A. D. Cole n finds A = 2.52 /, 

 for a Klemencic receiver 7 to 8 cm. long and 3.1 mm. in diameter. 

 This is in good agreement with Macdonald's theoretical relation. It is 

 doubtful, however, if Macdonald's equation, which was derived by con- 

 sidering the oscillator or receiver to be indefinitely thin in comparison 

 with its length, was intended to apply to the relatively thick receivers 

 of Cole's experiment. 



Another very admirable set of measurements with short oscillators 

 has recently been published by Webb and Woodman. 12 With an un- 



* Am. Jour. Sci., 1895, 49, 297. B Phys. Review, 1896, 24, 152. 



6 Elektrotech. Zeits., 1908, 29, 703. 7 Wied. Ann. 1898, 66, 435. 



8 Phil. Mag., 1904, 8, 105. 9 Electric Waves, 111. 



10 Phil. Mag. 1904, 7, 635. " Phys. Review, 1905, 20, 268. 

 " Phys. Review, 1909, 29, 89. 



