228 



PROCEEDINGS OF THE AMERICAN ACADERIY. 



800 



90 



Fig. 4. 



To determine the character of the vibration along the wire, the 

 lengths P Q and R S were fixed at 859 cm., the exploring terminals 



were moved along the rec- 

 tangle, and the bolometer 

 reading taken for each position 

 of the exploring terminals. 

 The graphic representation of 

 the results is shown in Figure 1 

 of the Plate. The character 

 of the curve indicates a simple 

 form of vibration. The total 

 length of the wire is equiv- 

 alent to seven half wave 

 lengths. The minimum points 

 are very nearly the same dis- 

 tance apart, and the distance 

 from the minimum occurring 

 at 748 cm. to the centre of the side Q R may be taken as three half 

 wave lengths. This furnishes a ready means of calculating the half 

 wave length ; — 



Q R =z 30 cm. 748 X 15 = 763 cm. 



763 -f- 3 — 254.3 cm. = a half wave length. 



The distance from this minimum to the end of the wire at P should 

 be one fourth wave length, or 127. 15fm. The actual distance is 

 859 — 748 =111 cm., so that the correction due to 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 ; for in finding the points of maximum 

 effect many trials had to be made, and the wire cut off a few centime- 

 ters at a time and then renewed many times. To remove this source 

 of inconvenience, the ends P and S were wound on wooden bobbins, 

 so that shortening and lengthening could be produced without cutting 

 the wire. This was a marked improvement, but the changing size of 

 the coils, as the wire was shortened or lengthened, varied the capacity 

 at the end slightly, and somewhat irregularly. This led to the adop- 

 tion of the arrangement shown in Figure 5. 



The secondary circuit consisted of the rectangle K L M N. The 

 side L N was open, and the lengths of the sides K L and M N could 

 be varied between a few centimeters and ten meters. The ends of 

 the wires K L and M N were in reality formed by the small copper 



