St. John — Wave lengths of Electricity on Iron Wires. 319 



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 42-5 45 50 60 75 



Deflections 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 1ST 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 accumula- 

 tion is least and the movement more unrestrained. The 

 electro-motive impulses from the vibrator act directly upon 

 the side KM so that remains a point of free motion or the 

 ventral segment of the wave, while L and 1ST 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 3 half wave lengths 

 long, and a third when the circuit is 5 half lengths and so on.* 

 It is known that the change of period produced by replacing 

 copper by iron does not exceed two per cent. The difference 

 in length between a copper and an iron circuit of the same 

 period would be very small with circuits a half wave length 

 long ; but this difference would be 3 times as great with 

 circuits 3 half wave lengths, long, and there might be a 

 cumulative difference that would become measurable by the 

 use of circuits of still greater length. To examine this ques- 

 tion, a copper wire (diameter O1201 cm ) was used as the second- 

 ary circuit in fig. 2a. The sides were taken 15 cm long and 

 gradually lengthened to 875 cm , and bolometer readings taken 

 for each addition, the exploring terminals being always at the 

 ends L and 'N. The result is shown graphically by the upper 

 curve in fig. 4, of the plate. The critical points in the curve 

 are the results of many separate determinations. The un- 

 steadiness of the spark in the vibrator made the determina- 

 tions somewhat laborious, though a single series of observations 

 would locate a maximum very closely. After this had been 

 done, the space of about a meter including the maximum 

 point was worked over forward and back ; the constancy of 

 the spark was assured by choosing a convenient point of 

 reference as already described. 



An examination of the curve shows four maxima occurring 

 when the sides of the rectangle were 45, 306, 562'5 and 818 



* J. J. Thompson, Eecent Researches in Electricity and Magnetism, p. 297 



cm 



