St. John — Wave lengths of Electricity on Iron Wires.. 315 



QR = 30 cm . 748 + 15 = 763 cm . 763 ~ 3 = 254-3 cm . = a half 

 wave length. The distance from this minimum to the end of the 

 wire P should be a fourth wave length or 127*15 cm . The 

 actual distance is 859 — 748 = lll cm , so that the correction for 

 the free end of the wire is about 16 cm . 



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 would 

 overbalance much inconvenience. To remove this source of 

 inconvenience, the ends P and S, fig. 2a, 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, varied the capacity at 

 the end slightly and somewhat irregularly. This led to. the 

 adoption of the arrangement shown in fig. 2a. 



The secondary circuit consisted of the rectangle KLMN 

 with the side LN open. The lengths of the sides KL and 

 MN could be varied between 15 cm , and 1000 cm . The ends were 

 really formed by the small copper boxes M and N. These 

 were 10 cm square and 4 cm 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 hold- 

 ing 10 m of the largest wire experimented upon. In the front 

 of each box was a small opening for the passage of the wire, 

 but to assure 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. 



Eor making and breaking the current through the induction 

 coil, an interruptor which would work with certainty and 

 regularity was much needed. With the assistance of the 

 mechanician of the laboratory I devised an interruptor which 

 gave very satisfactory results. 



A small electric motor (fig. 4a) was used to produce the 

 necessary motion, this was actuated by the current from two 



