562 REPORT — 1891. 



at any point. At about 4'6 m. there was a minimum deflection of 11, at 6-7 m. a 

 maximum of 46, at 9 m. a minimum of 13, at 11 m. a maximum of 23, and so on. 

 Tlius the waves tail oft' rapidly. There are two complete strongly-marked wares 

 and indications of a third, after which the disturbance tends to become steady along 

 the wire. 



IT. AVhen the far ends of the wires were joined together similar results were 

 obtained, excepting that the positions of maxima and minima were interchanged, 

 the disturbance, e.f/., being a maximum at the ends. The results indicate that only 

 a small number of waves are sent out by the primary vibrator, and that these are 

 rapidly damped. In both the above sets of experiments the primary and secondary 

 plates were 30 cm. apart. 



III. (Ends joined). On bringing the .secondary and primary plates nearer 

 together the damping became more and more rapid, as if the secondary more 

 quickly absorbed the energy radiated out by the primary. When the plates were 

 5 cm. apart only one wave could be detected. 



The curves (I, II, III) given above were measured on different days and under 

 different circumstances. They cease where the errors of observation become com- 

 parable with the variations to be measured. The author's method has the advantage 

 of reqiuriug only the simplest apparatus. The only other published method which 

 has been used for such measurements is Dr. Rubens' bolometric method, but Mr. 

 Bjerknes has obtained similar results to those of the author with an electrometer 

 instead of a thermo-junction. 



lu order to find whether the junction produced any disturbance, loops of wire 

 of varying lengths were inserted at 17 m. from the far end ; but loops up to 1 m. 

 long did not appear seriously to affect the positions of the maxima and minima. 



6. On the Froimgation of Electromagnetic Waves in Wires. 

 By Walter Thoep. 



The following is an account of some experiments made in the physical labora- 

 tory at Trinity College, Dublin, with apparatus kindly placed at the author's dis- 

 posal by Professor Fitzgerald. 



The experiments are incomplete, inasmuch as Mr. Trouton's value (0'68 metre) 

 of the wave-length in air is assumed for the resonating circle which the author 

 used. The author's determinations of this wave-length agree with Mr. Trouton's, 

 but thev were few in number, and made at the very commencement of the work. 



These experiments were undertaken with the hope of throwing some light upon 

 the results previously obtained by Professor Hertz.' He found the ratio of the 

 velocity of propagation of electromagnetic waves in air to the velocity in copper 

 wires to be as 75 : 47, or VQ. His wave-length in air was 7'5 metres. 



Using much shorter waves (0 68m.) and wires of different diameters, the author 

 obtained a ratio varying from 1 77 for very fine wires to very near unity for thick 

 wires. 



The apparatus has been fully described by Mr. Trouton.* The wire used was 

 soldered atone end to a piece of iron plate (9 x 4cm. and O'Scm. thick), which was 

 attached by means of silken cord to the vertical wooden support of the oscillators, 

 the plate being fixed opposite one of the cylindrical oscillators. The wire was sup- 

 ported hoiizontally along the axis of the parabolic cylinder used to concentrate the 

 radiations. 



In the first few experiments the further end of the wire was bent into a very 

 small hook, to which a piece of string was attached to keep the wire taut ; but 

 this minute hook was found to cause considerable disturbance at the end which 

 was never a node, and the distance from the end to the first node along the wire 

 was always less than the other internodes. The hook was therefore removed, and 

 the end of the wire kept straight. 



The resonating circle was 7ucm. diameter, and was held with its plane parallel 

 to the wire, and with the spark gap at its greatest distance from the wire. 



' Wiedemann's Annalen, vol. 34, p. 5.51. 

 ' Nature, vol. 39, p. 391 ; vol. 40, p. 398. 



