CHAPTER V. 



AN ELECTRODYNAMOMETER USING THE VIBRATION TELESCOPE. 



26. Introductory. The present paper is in the main contributory to the 

 much more difficult experiments of the next chapter. The employment of a 

 telescope with a vibrating objective did good service as an aid to the inter- 

 ferometry of vibrating systems. It seemed worth while, therefore, to see 

 what could be got out of it, when used in connection with a telephone only, 

 as a dynamometer. The experiments are of interest both because of the vibra- 

 tory phenomena observable and in view of the peculiar method of optic obser- 

 vation developed. Its particular use in finding the magnetic field within a 

 helix of any shape, but of unknown constants, deserves mention at the outset. 

 The attempt made in 36 to detect the node in an organ pipe, bolometrically, 

 may also be referred to. 



27. Apparatus. A front view (elevation) of the design is given in figure 41 

 and an enlarged detail (side view) in figure 42. The apparatus consists of a 

 rigid rectangular framework of 0.25 inch gas-pipe A, B, C, D,EE', F being 

 the foot attached to a tripod. There may be a steadying foot at C'. A and D 

 are attached to EE' by the stout clamps c, c", so that EE', lying behind the 

 plane of ABCD, admits of the attachment of a suitable clamp c', by which 

 the telephone, ih, may be secured in the same plane. B and c may be forced 

 apart slightly by the screw n, controlled by the broad thumb-nut m, the conical 

 end of n rotating in a socket of the cap p. 



The vibrating system consists of the bifilar wires of phosphor bronze or 

 steel dd', and the frame or carriage of the lens/, which is the movable objective 

 of the telescope T, the latter part containing the ocular and a plate micrometer 

 (centimeter divided in 100 parts). T may be at a considerable distance (50 

 cm. or more) from /, and supported by a convenient standard. The frame 

 of the lens (which must hold it securely, cement being used if necessary) is of 

 light sheet metal, the parts gg' being of sheet-iron (0.05 cm. thick), so as to be 

 attracted by the magnet i of the telephone. The stiff cross-wires r, s of the 

 frame are either soldered to the bifilar system dd' or otherwise attached to it 

 (soft sealing-wax does very well for temporary experimental purposes). 



The attachment and tension-control of the bifilar suspension is finally to be 

 described, as its period must be synchronized with the alternating current. 

 Results are obtainable only when the two periods are strictly in unison. In 

 figure 41 the wires dd' are looped around a groove in the pipe D below, and 

 the upper ends of dd', after passing a similar groove in A , are bent around the 

 posts a, a' , and wound respectively around the snugly fitting screws b, b', the 

 ends being secured against sliding by a fine hack-saw cut in the screws. To 

 stretch a wire it is passed from the notch in b once or twice around it, thence 



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