8 



ELECTRICAL MEASUREMENTS 



Suspension Fibre 



2. Magnetic shields may be employed. 



An astatic suspended system is an arrangement of two sets of 

 magnets of equal moment lying in the same plane and rigidly 

 connected. The north poles of the upper element are directly 

 over the south poles of the lower element. A perfect system 

 of this sort if placed in a uniform field will experience no directive 

 force, and it is necessary to resort to the use of a controlling mag- 

 net. This magnet is placed in a horizontal position, usually 

 above the system, and creates a field which is stronger at the 

 upper than at the lower needles. The con- 

 trolling force becomes less as the fields at the 

 upper and lower needles approach equality. 



It will be seen that fluctuations of H, the 

 local field, will affect the directive fields at 

 the upper and lower needles equally and have 

 no influence on the sensitiveness or the zero 

 reading of the instrument. 



In practice, it is not possible to adjust the 

 two sets of magnets so that they are exactly 

 in the same plane, nor is it possible to make 



Q their moments exactly equal; so the theoretic- 



ally perfect system cannot be realized. The 

 actual system will have a small resultant 

 polarity and when it is suspended the needles 

 will take up an approximately east and west 

 direction. Such a system will not be free 

 from the effects of variations of the local field. 

 Again, the magnetism of so-called permanent 

 magnets gradually deteriorates so that the 

 system cannot be expected to remain properly 

 magnetized even though its initial adjustment 

 For this reason, some better expedient has been 

 t and recourse has been had to magnetic shielding. 

 Magnetic Shields. 2 In order that a galvanometer may be 

 tively protected from variations of the local field by means 

 on shields, it must be of small size so that the volume of 

 the space to be shielded is reduced to a minimum. The shields 

 ither spherical or cylindrical; for mechanical reasons the 

 form 13 to be preferred. The cylindrical shields are left 



*- Straight Glass or 

 Quartz Shank 



Fio. 6. Astatic 

 needle system. 



is perfect. 



