﻿434 Mr. C. E. St. John on Wave-lengths 



exploring terminals, while with the plain wire it seemed dis- 

 tributed over a greater distance. In each case the effect of 

 the ends was to make the curve depart from the normal form 

 along the free wire. 



The theory of my investigations rests upon the principle of 

 electrical resonance. The "sides of the rectangle K L M N 

 (fig. 2, p. 428) were shortened to a few centimetres in length, 

 so that it could be safely assumed that the period of the 

 secondary was considerably shorter than that of the vibrator. 

 The exploring terminals were kept at LN, and bolometer 

 observations taken for each small addition to the length of the 

 sides KL and MN. When best resonance was found with 

 the shortest length of the secondary circuit that gave a maxi- 

 mum, it was assumed that the secondary had the same period 

 as the vibrator and that its equivalent length was a half wave- 

 length, its actual length depending upon the effect due to the 

 free ends. The occurrence of resonance is a very marked 

 phenomenon even with a vibrator that damps as rapidly as 

 the Hertzian. The accompanying table shows two series of 

 readings for the first maximum when an iron wire was 

 used : — 



Length of sides of rectangle... 15 25 35 40 423 45 50 60 75 



Deflexions of Galvanometer... 107 145 156 194-3 199-2 181-5 140 81 42 



... 94 119 161 185 191 178 136 76 34 



There can be no free motion of electricity at the ends of the 

 secondary circuit, but an accumulation alternately positive 

 and negative, and a resulting alternation of potential, the 

 phase at L being always opposite to the phase at N in case of 

 resonance. Elsewhere along the circuit the electricity moves 

 with more freedom and less accumulation. The point may 

 be called the electrical middle of the circuit, where the accu- 

 mulation is least and the movement most unrestrained. The 

 electromotive impulses from the vibrator act directly upon the 

 side K M, so that O remains a point of free motion or the 

 ventral segment of the wave, while L and N are always places 

 of no electric movement, or the nodal points. The shortest 

 circuit being a half wave-length, a second resonating circuit 

 ought to be found by increasing each side of the rectangle 

 by a half wave-length, making the circuit three half wave- 

 lengths long, and a third when the circuit is five half wave- 

 lengths, and so on*. It is known that the change of period 



* J. J. Thomson, l Recent Researches in Electricity and Magnetism,' 

 p. 297. 





