on Galvanometer 



67 



As the coils of a reflecting-galvanometer have to be sup- 

 ported by a central framework, there is necessarily a certain 

 distance left between the coils when they are in position. 

 This space we find when drawn to scale is that contained be- 

 tween the two vertical lines MM', and which therefore allows 

 more than sufficient room for the needle to turn. Hence if 

 the plan of opposite winding that we have suggested be tried, 

 the face of the coils may be made quite plane and no portion 

 whatever of the coils left unwound, even to allow the needle 

 space to move. 



III. Damping the Oscillations. 



Since no hole is left in the lower coil for the aluminium 

 damper, as is the case with Messrs. Elliott's reflecting-gal- 

 vanometers, a different mode of damping is adopted. The 

 plan we find best is to attach the mirror M (fig. 5) to the 



Figr. 5. 



middle of a long vertical strip of mica, S, across the upper 

 and lower ends and the middle of which the magnets m 1} w 3 , 

 and m 2 are fixed. Such an arrangement is also seen in 

 position in the galvanometer shown in figure 6 later on. 

 With this arrangement, which requires very little space, 

 we find that when the controlling magnet is adjusted so 

 that the periodic time of vibration is 7 seconds or more 

 the spot of light does not visibly pass through the zero 

 or the instrument is dead-beat, and even when the period 

 is much shorter than this, the spot of light does not pass 



F 2 



