TRANSACTIONS OF SECTION A. 



561 



of which the vibrators are made, being determined only by their forms, and there 

 is no considerable loss due to degradation into heat in these conductors. The 

 question occurs, what are the surface conditions that must be imposed under these 

 circumstances at the boundaries of the dielectric, in order that the vibrations may 

 be discussed with reference only to the dielectric in which they exist and are 

 propagated P 



It appears that the vibrations are analogous to those of an elastic solid, when 

 elastic displacement is made the analogue of the electric displacement in the dielec- 

 tric. It is demonstrable ' that if the velocity of propagation is the inverse square 

 root of the specific inductive capacity, this auxiliary solid must be considered as 

 incompressible, and the scheme of electrodynamics must be that of Maxwell. The 

 surface condition will then be absolute stiffness in the surface layer for all tano-en- 

 tial displacement, and freedom for normal displacement. 



The mathematical examination of a typical case shows that this way of pre- 

 senting the phenomena is practically exact for all wave-lengths greater than a cen- 

 timetre for copper or other good conducting metal. For very minute waves the 

 circumstances are not independent of the material of the conductor, but are similar 

 to those which actually exist in the case of the metallic reflexion of light-waves. 



By aid of this representation a qualitative view of the possible modes of vibra- 

 tion IS rendered feasible in cases where the mathematical analysis would beditficult 

 or impossible. '-^ 



5. On the Measurement of Stationary Hertzian Oscillations along Wires, 

 and the Damping of Electric Waves. By Professor D. E. Jones, B.Sc. 



An account was given of preliminary experiments made in Bonn (at the 

 suggestion of Professor Hertz) on electric waves in wires. The first object was to 

 find a smiple method of measuring the disturbance at different points of a wire (or 

 pair of wires) along which are sent waves which interfere after reflection at the 

 ends. It was found that satisfactory measurements could be made by inserting a 

 very small thermo-j unction in the circuit at diflerent points, and noting the deflec- 

 tion of a low-resistance galvanometer connected up to it. The method is delicate 

 enough to detect and measure exceedingly small currents, such as those produced 

 by telephones. 



The method was applied to measure disturbances along a pair of parallel wires 

 about 8 cm. apart and each about 130 metres in length. One end of each wire was 

 connected to a (secondary) metallic plate 40 cm. in diameter. In the first set of 

 experiments the other (far) ends of the wires were left free. The vibrator was of 

 the usual type, provided with plates of the same size as those on the near ends of 

 the wires and facing them. The wave-length of the disturbance along the wires 

 was about 4-3 metres. On plotting curves with distances from the (far) ends of 

 the wires as abscissae and galvanometer deflections as ordinates the following 

 results were obtained : — 



I. The disturbance was zero at the end (0) rising to a maximum (51) at 2'2 m. 

 There was no further absolute minimum, i.e. the disturbance did not fall to zero 



1891, 



> Proc. Bin/. Soc. May 1891. 



* Vroc. Camb. Phil. Soc. May 1891. 







