Mr Searle, Apparatus for Measuring the 'Extension of a Wire. 321 



a sensitive level L. One end of the level is pivoted to the frame 

 CD by the pivots H ; the other end of the level rests upon the 

 end of a vertical screw 8 working in a nut attached to the frame 

 CD'. The two links K, K' prevent the frames from twisting 

 relative to each other about vertical axes, but freely allow vertical 

 relative motion. When these links are horizontal the two wires 

 are parallel to each other. From the lower ends of the frames 

 CD, CD' hang a mass M and a pan P represented diagram- 

 matically in the figure. The weights of M and P are sufficient 

 to ensure that the wires are straight. The connexions between 

 the wires and the frames are made by the swivels F into which 

 the ends of the wires are soldered. The swivels enable the wires 

 to be without torsion and thus ensure that the two wires hang in 

 a vertical plane. Two other swivels connect M and P to the 

 frames. 



The head of the screw is divided, and a scale R engraved on 

 the side of the frame serves to determine the number of complete 

 revolutions made by the screw. In the instrument exhibited to 

 the Society the pitch of the screw is |mm. l , while the head is 

 divided into 100 parts. Each division on the head thus corresponds 



to ^o min - 2 



The instrument is used in the following manner. Suppose 

 that the screw has been adjusted so that one end of the bubble 

 of the level is at its fiducial mark. If a mass be placed in the 

 pan P, the wire A' is stretched and the bubble moves towards H. 

 The bubble is then brought back to its fiducial mark by turning 

 the screw so as to raise the end of the level resting upon it. 

 The distance through which the screw is moved is clearly equal 

 to the increase of length of the wire A' and is determined at 

 once by the difference of the reading of the screw in the two 

 positions. The level is sensitive enough to enable the screw to 

 be adjusted to -^ of a division on its head, i.e. to ^mm. 



To steady the instrument, it is convenient to allow the two 

 wires to press lightly against a rod fixed horizontally at a small 

 distance above the frames CD, CD'. 



A brief discussion of the kinematics of the instrument may 

 be added. In order to secure that there shall be only one possible 

 type of displacement of one frame relative to the other, five 

 degrees of relative freedom must be destroyed. Since only 

 relative motion is in question we may imagine one frame, say CD, 

 to be fixed. The other frame CD' is kept vertical by the tensions 

 of the wires above and below it. If the links were absent it 



1 A pitch of 1 mm. would be more convenient and would give ample sensi- 

 tiveness. 



2 The chief dimensions of the apparatus are as follows: — CD = 11 cm. Length 

 of links 5 cm. Diameter of screw-head =4 cm. 



