Size of Reflector in "Hertz's Experiment" 89 



24 cm. square, the node was found at 22*4 cm., a little further 

 than 21*8, which it is at when the sheet is long in the 

 magnetic direction (Table I.), and this is as it should be, 

 shifted out owing to both. Again, a reflector 24 cm. in the 

 electric direction, and 10 cm. in the magnetic, had a node at 

 21*5 cm., a little less than with a long sheet, and this also is 

 in the direction to be anticipated. Some few other experi- 

 ments were made, but in the absence of a workable theory 

 for the whole phenomenon, it was thought undesirable to 

 pursue the matter further at present. 



An interesting point arises in connexion with those ex- 

 periments made with the reflector narrow in the magnetic 

 direction, in which an inward shifting of the node was ob- 

 served. In Hertz's original determinations he found that 

 the first node was situated not quite so much as J the wave- 

 length from the reflector. He attributed this to imperfect 

 reflexion arising from imperfect conductivity in the material 

 of his reflector, which was a large sandstone wall with a 

 number of gas-pipes across it, and had on it besides to in- 

 crease the effect a zinc sheet 2 m. long in the magnetic 

 direction and 4 m. long in the electric; there were also wires 

 attached to the ends of the sheet for leading off the charges, 

 so that it must have been equivalent to a much longer one in 

 the electric direction. Now it seems probable that the sheet 

 was sensibly the only thing that reflected, and that the in- 

 ward displacement was really due to its comparatively small 

 size in the magnetic direction, for the wave-length he em- 

 ployed was nearly 10 m., so that the reflector was about 

 J the wave-length in width. This corresponds to about a 

 14 cm. mirror with a wave-length of 68 cm. 



Hertz found the inward displacement correspondingly 

 greater than is given in Table II., but his incident beam must 

 have been divergent, as he worked nearer the vibrator in pro- 

 portion to the wave-length, while my beam was rendered 

 fairly parallel by the parabolic mirror employed. 



To make quite sure that in reflexion off a large sheet the 

 first nodal distance 17 cm. was really one quarter the wave- 

 length, the position of the second node was redetermined and 

 found to be distanced from the reflector about 51 cm. — the 

 mean of five determinations*. The vibrator in these experi- 

 ments was over three wave-lengths away from the reflector. 



It is very convenient, in carrying out these experiments, to 

 have an arrangement for moving the resonator backwards and 

 forwards a known amount. A simple arrangement for effect- 

 ing this is to support the resonator at one end of a rod 

 * 2nd node.. U 49| | 51| 1 50| 1 50 | 51 |j 50-6 



