on Galvanometers. 



79 



We may here mention that the instrument shown in fig. 5 

 is spoken of in this paper as a galvanometer with " one pair of 

 coils," while those illustrated in figs. 6 and 7 are referred to 

 as having each " two pairs of coils/' 



Type 2 has been employed by Lord Rayleigh, the Profs. 

 Gray, Mr. Rosenthal, by ourselves, and others. Figs. 8 and 9 

 illustrate an instrument of this type, constructed by Messrs. 

 Edelmann of Munich, and called a Rosenthal Micro-galvano- 

 meter, since this instrument has the peculiarity that the coils 

 into which the ends of the magnet are sucked are extremely 

 small, having an outside diameter of only 11 mm. and a thickness 

 of 2 mm. Fig. 9 shows the suspended magnet of the Rosenthal 

 galvanometer as constructed by the makers, the arrangement 

 being non-astatic ; but we have also made and used with this 

 galvanometer an astatic combination, constructed as in 

 % 10. 



Fig. 10. 



Mirror and astatic pair of needles for the Kosenthal Micro-galvanometer. 



Figs. 11 and 12 show, in plan and elevation, a ballistic gal- 

 vanometer constructed for the Central Institution by Messrs. 

 White of Glasgow, from drawings made by us in consultation 

 with Prof. T. Gray. The needles ns, ns, which are arranged so 

 as to produce an astatic combination, are each at one end 

 sucked into a coil and at the other end pushed out of a coil, the 

 coils being contained in four small, rectangular, hinged boxes, 

 B, which are kept closed by spring catches. The vertical 



