on Galvanometers. 



75 



Ordinary d'Arsonval Galvanometer. 



Deflexion from one 



Relative Strength 



Error. 



Percentage 



end of Scale. 



of Current. 



Error. 

















60 



59 



-1 



-1-7 



120 



118-5 



-1-5 



-13 



180 



180 











240 



240-6 



+0-6 



+C-2 



300 



300 











360 



359-8 



-0-2 



-0-06 



420 



416-5 



-3-5 



-0-8 



480 



473-5 



-6-5 



-1-4 



540 



537 



-3 



-06 



600 



600 









We find that it is possible, however, to obtain with a 

 d'Arsonval galvanometer a far more exact proportionality 

 between the deflexion and the current than is given in the 

 preceding table by: — 



Supporting the coil independently of the iron core, so that 

 fairly accurate centering becomes possible. This result 

 has been to a certain extent accomplished in the small 

 type of d'Arsonval galvanometer constructed by Messrs. 

 Nalder. 



Allowing the coil to hang freely from the top suspension 

 so that the centre of gravity of the coil is always in the 

 axis of rotation. Electric continuity at the bottom of 

 the coil is then maintained by a spiral of fine wire 

 offering little torsional rigidity. 



Fitting iron pole-pieces to the stationary magnets, and 

 shaping these pole-pieces so that the rate of cutting lines 

 of force by the moving coil is practically constant. 



The following gives the results of tests made on a d'Arsonval 

 galvanometer constructed in this way, each divisional length 

 on the scale being 1*05 millim., so that the total deflexion of 

 600 divisions represented a motion of the spot of light over 

 about 63 centimetres, or about 25 inches. The deflexion 

 could be read easily to 0*2 of a division, and it will be ob- 

 served that the greatest want of proportionality is only 

 0*15 per cent., which occurs with a deflexion of 480 scale- 

 divisions. 



