32 



ELECTRICAL MEASUREMENTS 



Fig. 14; it is seen that the reading is most quickly obtained when 

 the instrument is very nearly critically damped. The deflec- 

 tion of a critically damped galvanometer is within Ko.per cent, 

 of its ultimate value, at a time which is approximately equal 

 to 1.5T , and within 1 per cent, at a time which is approxi- 

 mately equal to T . 



s 



.08 .10 02 04 06 08 .20 .22 .24.5)6 .28 .30 .32.34 .3G .38 .40 .42 .44 .46 .48 .50 .52 .54 .56 .58 .* 



'Dair "ing Coustaat /j 



Fio. 14. Showing the effect of damping on the time required by a galvan- 

 ometer to attain its deflection. 



The D'Arsonval or Moving-coil Galvanometer. 7 Sir William 

 Thomson used the suspended-coil principle in his siphon re- 

 corder (1870) and its application to galvanometers was later 

 suggested by Maxwell in his Treatise on Electricity and 

 Magnetism. 



The name D'Arsonval is frequently applied to galvanometers 

 of this class, attention having been recalled to them by Deprez 

 and D'Arsonval in 1882. 



The great practical advantage of this instrument lies in its 

 freedom from the effects of stray fields and the ease with which a 

 long and uniform scale may be attained. It is these things that 

 have caused it to be adopted, in a modified form, for direct- 

 current ammeters and voltmeters. 



The essential features of a moving-coil galvanometer are 



shown in Fig. 15. The field is furnished by a strong permanent 



mgnet and the movable coil swings in the air gaps between the 



* of the magnet and a fixed iron core. The coil is hung by a 



fine wire suspension which also serves as a lead, while below the 



