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



QB =30 centim. 748 + 15 = 763 centim. 763-=-3=254'3 

 centim . = a half wave-length. The distance from this minimum 

 to the end of the wire P should be a fourth wave-length of 

 127*15 centim. The actual distance is 859 — 748 = 111 centim., 

 so that the correction for the free end of the wire is about 

 16 centim. 



To adjust the length of the wire under this arrangement was 

 a work of considerable difficulty, but the possibility of using 

 a single-wire circuit free from disturbing capacities over- 

 balanced much inconvenience. To remove some of this, 

 the ends P and S were wound upon wooden bobbins, so that 

 shortening and lengthening could be produced without 

 cutting the wire. This was a marked gain in convenience ; 

 but the changing size and form of the coils, as the wire 

 was shortened or lengthened, altered the capacity at the end 

 slightly and somewhat irregularly. This led to the adoption 

 of the arrangement shown in fig. 2. 



The secondary circuit consists of the rectangle KLMN 

 with the side L N open: the lengths of the sides K L and M N 

 can be varied between 15 centim. and 1000 centim. The 

 ends are really formed by the small copper boxes L and N. 

 These were 10 centim. square and 4 centim. thick, and 

 mounted upon the wooden bar E by insulating supports. 

 Within the boxes were wooden bobbins fixed on a hard rubber 

 axle, and each capable of holding 10 metres of the largest 

 wire experimented upon. In the front of each box was a 

 small opening for the passage of the wire, but, to insure a 

 firm contact between the wire and the boxes, a brass block 

 was soldered on the inner side of the front and a binding 

 screw passed in from the side of the box. The bar E was 

 fastened to a wooden support resting upon a car which ran 

 on a wooden track extending the entire length of the room. 

 The car carried a brake so that the wires could be drawn taut, 

 and the wooden screw held the axle from turning. With this 

 arrangement the length of the wire could be varied at pleasure 

 while the end capacities remained constant. 



The end capacities are not a desirable feature, for their own 

 sake, since they destroy the perfect simplicity of the plain 

 rectangular circuit, and seem to detract somewhat from the 

 sharpness of the maxima ; but the gain in convenience and 

 the possibility of obtaining a large number of observations 

 whose average values can be used may overbalance these 

 considerations. 



For making and breaking the current through the induction- 

 coil, an interruptor which would work with certainty and 



