12 ELECTRICAL MEASUREMENTS 



Symmetry is essential in order that mechanical vibrations may 

 not produce excessive disturbances of the needle system and thus 

 render difficult the reading of the instrument. The moment of 

 inertia of the non-magnetic parts of the system should be reduced 

 to a minimum, so the mirror must be small and light. It may 

 be cut from a (silvered) cover glass such as is used on microscope 

 slides. 



Each member of an astatic system may consist of six or seven 

 magnets. Glass-hard tungsten steel is used for the magnets 

 which may be about 1.2 mm. long and 0.2 mm. or less in diameter. 

 The magnets are attached to the shank by minute drops of shellac. 

 To preserve the symmetry of the system the magnets are placed 

 on both sides of the shank. The total mass of a system con- 

 structed with great care, for use in research work, may be as 

 small as 6 or 7 mg. Such a very light system will be over-damped 

 by the air friction. 



Damping. In order to economize time, all galvanometers 

 should be properly damped so that they will come promptly to 

 rest. The various devices used, such as damping vanes or in 

 moving coil galvanometers, damping loops, are arrangements for 

 quickly dissipating the energy of motion of the movable 

 system. 



Arrangement of the Galvanometer Coils. In order to attain 

 a high sensitivity the windings must be so disposed that they 

 produce the maximum field at the needle. Having given a defi- 

 nite length of wire of a certain size, it will be most effective 

 when used on an astatic galvanometer. For suppose it to be 

 wound in a coil adapted to a single-needle instrument, the outer 

 layers will be at a considerable distance from the needle and 

 therefore their effectiveness will be small. If an astatic system 

 is used, these outer layers may be taken off and wound in coils 

 which closely surround and act on the lower member of the needle 

 system. Thus the wire as a whole is brought into a more advan- 

 tageous position and the sensitivity of the instrument increased . 



An additional advantage is derived from winding the wire on 

 four rather than on two spools. For, to a certain extent, the 

 galvanometer resistance may be changed to suit the work in 

 hand by connecting the four coils in series, series-parallel or in 

 parallel. 



