w 



the Investigation of Young's Modulus for Wires. 541 



(a, in fig.) has stretched more than the others, i. e. that 

 that wire has had more than its share of the Fig. 2. 



weight. To remedy this the upper and 

 lower ends of b and c are moved along their 

 respective slots towards the centre, and thus 

 get more of the stretching-force which is 

 applied at the centre of the lower disk, while 

 the upper disk is also suspended centrally. 

 In this way it is easy to adjust the three wires 

 so that, on stretching, the two disks move 

 parallel to each other ; and if a rod bearing 

 the prism be suspended from the top disk 

 and the mirror on its levelling-screws be set 

 on the bottom one, the centre of the inter- 

 ference-rings, having been got into the field, 

 w T ill remain there throughout the extension, 

 and the number of bands passing any fixed point in the field 

 will give the amount of extension in half-wave-lengths of the 

 light used. 



In such an arrangement the bending of the disks under the 

 stress would be superposed on the displacement due to 

 stretching of the wires, and in addition any slipping of the 

 wires in the clamps of the T-pieces would affect the observed 

 extension. To obviate any such " end effects " two thinner 

 disks of brass, each with three radial slits just wide enough to 

 admit free passage of the wire, were supported, one near the 

 upper end of the wires and the other near the lower end, each 

 on three clips, which had sharp edges to their hemispherical 

 heads so that the wires should be clipped at well-defined 

 points. From the upper of these the 

 prism-rod hung centrally, and on the 

 lower the mirror rested on three pegs 

 attached to springs by means of which 

 the levelling-screws could give fine ad- 

 justment, The coarse adjustment was 

 made by two screws on the prism- rod 

 which moved the prism in altitude in 

 two vertical planes at right angles. The 

 prism-plate and mirror-plate were ad- 

 justed horizontally when the clips were 

 put on. Fig. 3 shows a diagrammatic 

 section of the apparatus. 



Stretching-Force. — It was necessary 

 that the stretching-force should be ap- 

 plied gradually and without vibration. 

 This end was attained by making a hollow cylinder of tin- 



