198 Mr. G. F. C. Searle on the Elasticity of Wires. 



sufficiently small the damping becomes very small, indicating 

 that the elastic limit is not overstepped. With a steel wire 

 31'4 cms. in length and '0698 cm. in radius, 1 find it easy to 

 count 100 vibrations of one of the bars with an initial deflexion 

 of not more than 1°, the elasticity being practically perfect. 

 This result might be expected, for in this case the greatest 

 stress experienced by any part of the wire is only about r -^ of 

 the breaking stress. 



The suspending strings must be long and fine, so that the 

 torsional couples due to them may be negligible. 



Since any departure from the circular form of section of 

 the wire has an important influence upon the time of vibration 

 in the determination of E_, it is advisable to mark one of the 

 clamping- screws, and to take two sets of observations for the 

 period with the mark (1) vertical, (2) horizontal. The mean 

 of the two values is to be used in the formulae (3) or (11). 

 The two corresponding diameters should be measured by a 

 screw- gauge and the mean taken, or the mean diameter should 

 be found by the hydrostatic balance. 



The form of clamping-screw shown in fig. 2 is convenient ; 

 for if a number of wires of various metals are provided with 

 clamping-screws, the same inertia-bars serve for all the wires. 

 If short wires are used, the clamping-screws should be recessed 

 into the bars so that the end of the screw may be in the centre 

 of the bar, as shown in fig. 2 *. The wires are soldered into 

 holes drilled into the screws. 



The only remarks necessary with regard to the determination 

 of the rigidity are that small arcs of vibration should be used, 

 and that the suspending-hooks should be so light that their 

 moments of inertia about the axis of the wire are negligible. 



The masses of the bars should be determined, and, for 

 convenience, marked upon the bars before the holes are drilled 

 in them. 



The dimensions of my apparatus are as follows: — 



Length of each bar =32*1 cms. 



Width of each bar (square in section) =1'25 cm. 



Mass of each bar = 441 grammes. 



Moment of inertia of each bar= 3*793 X 10 4 grm. cm. 2 



Length of wires under test = 31*4 cms. 



Diameter of wires tested = *l to *15 cm. 



The following are some of the results obtained, the unit for 

 each modulus being one dyne per square centimetre. In 

 each case the value of E has been corrected for the mass of 



* This is not the case in my apparatus ; the end of the screw is about 

 •6 cm. from the centre of the "bar. 



