244 ADVANCED ELECTRICITY AND MAGNETISM. 



antinodes and the maximum generator voltage G must satisfy 

 the equation : 



r v 27rd 

 G = Esin-T 



A 



128. Experimental demonstration of standing waves on a 

 pair of straight wires. The velocity of progression of an electro- 

 magnetic wave on a plain transmission line is the same as the 

 velocity of light, namely, 186,000 miles per second, and therefore 

 it is necessary to use an alternating electromotive force of ex- 

 tremely high frequency if one is to produce a standing wave train 

 upon a plain transmission line of moderate length. The arrange- 

 ment used by Lecher is shown in Fig. 185. Two flat plates of 



metal C and D together with the rods r and the air gap g 

 constitute a Hertz oscillator which is excited from the induction 

 coil //; and C'D' are two auxiliary metal plates which are 

 connected to the long wires ABA' B r . A rapid reversal of elec- 

 tric charge on the plates C and D (produced by the electric 

 oscillations of the system CrgrD) induces reversals of charge 

 upon the plates C' and D', and the result is that a train of 

 waves is sent out along the pair of wires and reflected from its 

 distant end, thus producing a standing wave train. The voltage 

 distribution over the transmission line ABA'B' may be investi- 

 gated by bridging a small vacuum tube across the line as shown 

 at T in Fig. 185. When this vacuum tube is slid along the 

 wires it shows no luminosity at or near the voltage nodes, whereas 

 it shows a maximum luminosity at or near the antinodes. The 

 best form of vacuum tube for this purpose is shown in Fig. 186, 

 the narrow portion of the tube should have a bore of about one 



