somewhat analogous to Newton's Eings. 69 



FIG. 1. Diagram of Apparatus. 



EXPLANATION OF FIG-. I. 



I. Induction coil worked by two secondary cells. This coil could give 

 regular sparks up to 2 cm. long ; their frequency was about 25 per 

 second. 



G-. Spark gap, usually 2 mm. 

 PGP' was 204 cm. measured along the semicircle. The wires PG, G-P' were 



2 mm. diameter. 

 PP'. Condenser plates of zinc 40 cm. diameter to form the ends of the 



Hertzian primary oscillator. 



SS'. Similar plates at a distance of 30 cm. from P a/nd P', a.nd forming the 

 beginning of the long secondary which consists of copper wires about 

 1 mm. diameter. Those parts shown dotted in the figure were set up 

 . in the corridors adjoining the laboratory. With the above dimensions 

 waves 9 mm. long were produced, their frequency being about 33 

 millions per second. 



A A' = BB' = CC' = DD' = 8 cm. This distance was maintained throughout 

 the secondary by pieces of dry wood at intervals of about 2 or 3 

 metres. 

 iJCB'C'. The abnormal (or altered) part of the secondary used to produce the 



reflexion and interference phenomena. 

 MM'. The middle of the abnormal part. 



EE'. The electrometer. The needle is uncharged : it therefore turns in the 

 same direction whenever there is any potential difference between E 

 and E', whatever the sign of that difference. The needle was sus- 

 pended by a quartz-fibre, and gave (though uncharged) a deflexion of 

 about 35 scale divisions when the electrometer was connected to a 

 battery of 10 Daniell's cells. In using the electrometer for the 

 electric waves first throws, not steady deflexions, were always read, 

 DD'. Wire bridge across the main wires. 

 FH = HB = ED = a quarter of a wave-length. 



Hence, as the length of the abnormal part is gradually increased, 

 the total energy of the reflected waves is thereby periodically in- 

 creased and decreased. It was thus seen that the length of the 

 abnormal part of the secondary corresponds to the thickness of the 

 air film in the optical phenomena known as Newton's Rings, and 

 that the reflexions of the electrical waves at the beginning and end 

 of the abnormal part correspond respectively to the reflexions of light 

 at the first and second faces of the air film. 



For the experiments now to be described the results of the mathe- 

 matical theory previously developed* are as follows : *- 



* Loc. cit., equations (13), p. 90, and equations (18), p. 92. 



