74 



Messrs. Ayrton, Mather, and Sumpner 



be taken at one end of the scale and the spot of light deflected 

 right across the scale for increasing currents, since in this 

 latter case a variation in the law occurs as the plane of the 

 needles passes through the symmetrical position. This want 

 of proportionality is exhibited by the following figures : — 



Ordinary Reflecting-Galvanometer. 



Deflexion from one 



Relative Strength 



Error. 



Percentage 



end of the Scale. 



of Current. 



Error. 

















60 



620 



4-2-0 



3-4 



120 



1210 



+ 1-0 



0-8 



180 



181-5 



4-1-5 



09 



240 



2425 



4-2-5 



1-0 



300 



303-0 



4-30 



1-0 



360 



363-5 



4-3-5 



10 



420 



423-5 



4-3-5 



0-8 



480 



483-0 



+3-0 



0-7 



540 



542-0 



4-2-0 



0-4 



600 



6000 









This defect is even more marked in some parts of the scale 

 with the ordinary d'Arsonval galvanometer when the zero for 

 no current is at one end of the scale. This instrument, as 

 usually made by Messrs. Carpentier, has a tubular iron core 

 supported from a standard which carries the top suspension of 

 the coil, and the stationary permanent magnets are not pro- 

 vided with pole-pieces. In consequence of the core and the 

 suspension not being supported independently, there is great 

 difficulty in centering the coil relatively to both the core and 

 the stationary magnets ; further, neither the core nor the coil 

 can be readily removed. And partly because of this difficulty 

 of centering the coil, partly because its centre of gravity is 

 generally not in the line of suspensions, and partly because 

 the stationary magnets have no properly shaped pole-pieces, 

 there is (as may be seen from the following sample of the 

 results of many experiments we have made on d'Arsonval 

 galvanometers) an error in different parts of the scale far less 

 regular in its value than in the case of the ordinary reflecting- 

 galvanometer with stationary coil. 



