the Construction of Galnanometers, tjr. 235 



greatly adding to the strength of the frame. The coil is wound round the out- 

 side of the frame and its brass plates, leaving the internal circular chamber of 

 brass for the uninterrupted oscillations of the enclosed needle. Its vertical width 

 is reduced by the two brass plates to one-eighth of an inch. 



But the upper bar of the needle would still be liable to obstructions from 

 filaments of the upper layer of coil, were it not that a circular plate of well- 

 hammered silvered brass lies loosely on it, and exactly fills up the interior of 

 the circle on which the graduation is engraved. 



In order to permit the compound needle to be placed within, and removed 

 from its berth in the chamber, as occasion may require, there is a slit along the 

 middle of what I have (for want of a better name) called its ceiling, so narrow 

 as barely to admit the lower bar of the needle. To keep the wire of the coil 

 and its filaments in situation, without encroaching on this slit, its edges are 

 guarded all round by an elevated margin* of very thin sheet brass, which 

 stands exactly as high as the thickness of the coil. The circular silvered brass 

 plate above mentioned, although described as one piece, really consists of two 

 semicircles, the diametrical junction of which is so accurately fitted, that it ap- 

 pears as a straight engraved line upon one circular plate: it represents the 

 magnetic meridian of the galvanometer. When the needle is to be removed, the 

 semicircles are easily pushed up from below. Between the semicircles, and in 

 their common centre, there is a small hole, which permits the spindle or con- 

 necting axis of the two bars of the compound needle to play freely. In the top 

 of this spindle is a very small hole, barely large enough to receive the silk fibre 

 which sustains the compound needle. Thus the two bars, which constitute the 

 compound needle, are brought as near as possible to the three faces of the coil, 

 without risk of obstruction. The chamber in which one bar of the needle rotates 

 may be as vertically narrow as will allow free motion ; one-eighth of an inch will 

 be sufficient. In so narrow a space, it is obvious that any deviation from the 

 horizontal position of the coil and frame would cause obstruction to the move- 

 ments of the needle : the same would happen also if the needle were not accurately 

 balanced on its spindle, and therefore on the suspending fibre of silk. The 



* The coil must be wound as closely as possible against this margin on both sides ; for here the 

 energy of a voltaic current is most required to overcome the inertia of the needle. 

 VOL. XXII. 2 I 



