62 



Messrs. Ayrton, Mather, and Sumpner 



IT. Position of Mirror. 



According to the ordinary method of constructing a 

 Thomson's reflecting-galvanometer, a tubular space at the 

 centre of the coil is left unwound, and the mirror, with the 

 magnets at the back of it, hung at the centre of this unwound 

 space (see fig. 2, which illustrates a well-known form of this 

 type of galvanometer). To allow the ray of light to pass out 



Fig. 2. 





"•••*••/•*•»••*»•••••••• " i 



of the coil after reflexion at the mirror, when the mirror is 

 deflected, the end of the tubular space has to be made 

 trumpet-shaped, which leads to still more space in the 

 neighbourhood of the needle being left unwound. And when 

 the galvanometer is an astatic one, a somewhat similar tubular 

 space is left unwound on each side of the lower coil to allow 

 room for the diamond-shaped aluminium vane to turn. 



In this way the most valuable part of each coil, viz. that 

 close to the magnetic needles, is left unwound. This disad- 

 vantageous mode of construction so impressed itself on Mr. 

 Mudford, while a student at the City Guilds' Institute, that 

 he suggested that the needles alone should be placed within 





