Dr Searle, A bifilar method of measuring the rigidity of wires 63 



II 



§ 3. Ap'paratus. This is shown diagrammatically in Fig. 2. 

 The wires are soldered into torsion heads S, T, which pass through 

 a board XY held in a firm support. 



The lower ends of the wires are soldered into screws which pass 

 through "clearing" holes in the bar EF, and are secured with nuts. 

 The heads of the screws are made with "flats" to fit a spanner. 

 Before the screws are secured to EF, the torsion heads are set to 

 zero; the screws are then secured to EF so that, when the bar is 

 only subject to the action of the wires and of 

 gravity, the flats on both screws have the same 

 directions as when the wires hung freely. 



The distance BD is, as near as may be, equal 

 to AC. 



The load is carried by a knife-edge forming 

 part of the hnk iV, Figs. 2, 3. The knife-edge rests 

 in a V-groove in a plate, P, fixed to EF by screws 

 passing through slots. By adjusting P, the tensions 

 can be equalised; the notes emitted by the wires 

 when plucked have the same pitch when the ten- 

 sions are equal. 



A weight W (a few kilogrammes) is suspended by the rod Q 

 from the link N. A slot in the lower cross-piece of N allows Q to 

 be put into place; the nut drops into a recess. The weight should 

 be so attached to Q that it cannot turn about a vertical axis 

 relative to Q with any freedom; otherwise it will be difficult to 

 reduce the system to rest. 



The bar may be fitted with two pointers K, L, and the readings 

 of their tifs are taken on two horizontal 

 scales. These scales are adjusted to be 

 perpendicular to KL when the torsion 

 heads read zero. If KqLq is the straight 

 line through the zero positions of the tips 

 and K, L are the tips when the bar has 

 turned through d, Fig. 4 shows that 



Fig. 3. 



Fig. 4. 



sm U = 



KH _ yi+j/a 



LK 



V 



•(2) 



where y-^ = KKq, y^ = LLq and p = LK, the whole length of the 

 pointer system. 



The deflexion of the bar is best observed optically. A metal 

 strip R is screwed to EF, packing pieces being interposed to allow 

 the link N free movement, and a plane mirror is fixed to R. The 

 deflexion can be observed by aid of a telescope and scale, or of a 

 lamp and scale. It is, however, simpler to employ a goniometer 

 such as those which have been in constant use at the Cavendish 

 Laboratory for several years. A description of the instrument and 



