OF ELECTRO-DYNAMIC FORCES. 493 



of the support when it is thrown back. When the specuhim 

 //■ is now placed at g, and its counterpoise A A at i, the reel is 

 prepared for suspension in the multiplier by the metallic threads. 

 For this purpose both the supports of the reel terminate at e 

 and c' in hooks or pieces in the form of T, and the bifilar me- 

 tallic threads are furnished below with a small ivory beam, 

 II, which towards each end terminates in a metallic plate, 

 and this again in a small metallic cylinder ; the latter fit into 

 the above hooks or upsila of the support, and thus receive the 

 reel. The bifilar metallic threads n o and n' o' are united to the 

 cross-beam II in the following manner. The commencement n 

 of the thread no is fastened by means of a screw to the metallic 

 plate )', proceeds a short distance towards I, and then returns 

 through a small hole at the end of the plate beneath the beam II 

 to its centre j9, where it runs through a small hole again above the 

 beam, and can then be continued to o and further. The thread n'o' 

 is arranged in the same manner, its direction however being 

 reversed ; in the centre p of the beam / 1 each has a separate 

 aperture, through which it passes ; these lie very near each 

 other, but are separated and kept isolated by the ivory. The 

 index qq is placed upon the centre of the beam before the me- 

 tallic threads n o and w' o' are inserted. 



Fig. 2 exhibits the lateral view of the vibrating reel, with the 

 coil as placed upon the beam, and the mirror and counterpoise 

 adapted and vibrating on the bifilar metallic threads. Only the 

 very narrow anterior portion of the index is perceptible. 



Fig. 3 represents the reel seen at right angles to the surface 

 of the speculum ; the hooks or upsila, as also the index vibrating 

 above the scale-plate c c, are very distinctly seen. 



Fig. 4 presents the view from above, in which the beam and 

 the index form a right-angled cross. 



Fig. 5 serves to illustrate the further course of the bifilar 

 metallic thread to its termination ; for the sake of distinctness it 

 is represented of twice the size of the other figures, and as seen 

 in a vertical section. The bifilar metallic threads continue to 

 ascend from o and o', inclosed in a brass tube ; they are wound 

 round the moveable rollers a and «', and are finally fixed to the 

 ivory roller B at 6 and h' round rotating pegs. The threads can be 

 wound up or unwound on these pegs or small rollers by means 

 of a small key, according as the weight of the vibrating reel 

 may render this requisite ; the small rollers a and a' are also ne- 

 cessarily turned round at cither of these operations. The ivory 



