Galvanometers of High Sensibility. 



261 



mounted, one on each side of the staff, thus increasing the 

 number from eight to twenty; this would require shorter 

 magnets for maximum sensibility. 



We have not yet exactly realized this system in practice, but 

 give the result obtained with a system having only six magnets, 

 of the same material and cross section as I 2 , and an average 

 length of about l'l6 mm ; the non-magnetic moment of inertia 

 was probably a little less than that assumed in the above 

 calculation. The sensibility actually obtained, and also the 

 sensibility reduced to the Ayrton-Mather scale, is given in the 

 following table, together with the sensibilities of a few other 

 recent galvanometers, taken from the table of Ayrton and 

 Mather (Phil. Mag., Oct., 1898). 



It should be noted that with these very light systems the 

 " period " when it is at all long — say 5 s to 10 s complete — is 

 very difficult to determine, on account of the great damping. 

 Moreover, it is unfair to compare systems which were tested at 

 considerably different periods, and reduced to the same period 

 by assuming the sensibility oc T 2 — for this is on the basis of a 

 free period." In computing moments of inertia for the 

 above systems, we have assumed that the axis of rotation passed 

 through the center of gravity of the system and was parallel 

 to the staff ; if the system is badly constructed so that the axis 

 of rotation departs •2 mm from the staff at its upper extremity, the 

 moment of inertia will of course be increased, and the sensi- 

 bility (for a given period) decreased by about 25 per cent. The 

 moments of inertia of the four systems computed for use in 

 plotting curve 2 varied between 44X10-" and 529 XlO -8 

 C. G. S. units. 



Table. 



Galvanometer and Con- 

 stants when tested. 



Snow (Wied. Arm. 

 xivii. p. 218; Phys. 

 Rev., i, p. 37). 



4 coils, 30mm. ext. 

 and 6mm. int. diam. 

 Each coil wound 

 in 2 sections, with 

 1800 turns. 

 R = 140 ohms 



(series) 



Suspension System. 



6 small watch- 

 spring magnets in 

 each group. 3 on 

 each side of staff. 

 Magnets 3-4 mm. 

 long. 



Mirror 5 mm. 

 dia.. 0-14mm. thick. 



Quartz fibre 40cm. 

 long. 



Wt. of system = 

 80mg. 



Sensibility and condi- 

 tions of use when 

 tested. 



C=l 



10- n ( 



amp. 

 per mm. 

 Scale dist.=300cm. 

 T (complete)— 



10 sees. 

 11=140 ohms. 



Deflections per micro- 

 ampere when scale 

 dist.=iooo scale divs., 

 complete period = 

 io sees. Resistance 

 =i ohm. 



470 



See Ayrton and Mather, Phil. Mag., pp. 366, 1898. 



