332 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Also the presence of the spark-gap in the oscillator seems to be 

 without influence, as the values of Conrat were obtained for rods 

 without a gap. 



In discussing the question, raised by Pollock and Close, 15 as to 

 whether a result obtained for an infinitely thin ellipsoid can be applied 

 to an infinitely thin rod of uniform section, Lord Rayleigh 16 says : 

 " It appears therefore that the wave-length of the electrical vibration 

 associated with a straight terminated rod of infinitesimal section is 

 equal to twice the length of the rod, whether the shape be cylindrical 

 so that the radius is constant, or ellipsoidal so that the radius varies 

 in a finite ratio at different points of the length, and that this conclu- 

 sion remains undisturbed, even though the shape be not one of revolu- 

 tion." Lord Rayleigh, however, raises the question whether a sufficient 

 reduction of the diameter of the rod to comply with Abraham's ap- 

 proximation is experimentally possible without too greatly diminishing 

 the conductivity, which is assumed perfect in the theoretical discussion. 



In reply to this note by Lord Rayleigh, Macdonald 17 expresses the 

 view that the rate of damping of the free vibration associated with the 

 terminated straight wire is very large, and in fact not far removed from 

 the order of magnitude of the known result for a spherical vibrator. 

 This large damping, if it exists, and especially if it is due to a large 

 radiation from the wire near the ends, would account for a distortion 

 of the current distribution in the conductor so as to give a wave-length 

 larger than twice the length of the conductor. 



Since the question of the conductivity of the wire and the damping 

 of the oscillations has a bearing on the question of its period, it is pro- 

 posed to give the results of a measurement made on the damping of 

 one of the oscillators used in the present experiments. 



Damping. — The damping factor of a rectilinear oscillator 14 meters 

 long, consisting of two oppositely-extending horizontal wires 7 meters 

 long and 1 mm. in diameter, was determined by a method recently 

 given by K. E. F. Schmidt. 18 The spark-gap was 3 mm. long. 

 Schmidt's method consists in determining the average square current 

 in a low resistance wavemeter circuit for various adjustments of the 

 wavemeter in the neighborhood of resonance. To get the mean square 

 current in the wavemeter circuit the dynamometer shown in Figs. 2 

 and 3 was employed. The deflections of this instrument have been 

 shown to be proportional to the square of the current. The values ob- 

 tained are recorded in Table IV, which gives the wave-length adjust- 



15 Loc. cit. 16 Loc. cit. 



» Phil. Mag., 1904, 8, 276. " Phys. Zeits., 1908, 9, 13. 



