62 ALTERNATING CURRENTS 



lower edges will always have the same magnetic polarity, but 

 when the upper edges are north poles, the lower edges are south 

 poles. Therefore, there will always be a repulsion between the 

 two upper edges, and also between the two lower edges of the 

 iron strips. This repulsion tends to move the spindle against 

 the action of two springs. A pointer mounted on the spindle 

 moves over a graduated scale and indicates the voltage. 



This type of instrument can be used for direct current with a 

 precision of 1 or 2 per cent. Its obvious 

 advantages are its simplicity, its cheap- 

 ness, and the fact that there is no current 

 carried to the moving element. When 

 carefully calibrated, a precision of 0.5 per 

 cent., and better, can be obtained with 

 alternating current. This type of instru- 

 ment cannot be calibrated accurately 

 with direct current on account of the 

 FIG. GO. Inclined coil, effect of hysteresis on the vanes. It 

 iron-vane type of instru- should be calibrated by comparison 

 with an alternating-current standard. 



Air damping is obtained by the use of a light aluminum vane 

 moving in a restricted space. 



The iron-vane principle has been applied to the inclined-coil 

 type of instrument. A small iron vane, mounted obliquely on the 

 spindle, Fig. 60, replaces the inclined moving coil of Fig. 51 , page 54. 

 When the pointer is at zero, this vane lies at an angle to the coil 

 axis, as at a, Fig. 60. When current flows in the coil, the vane 

 attempts to take such a position that the direction of its axis 

 shall coincide with that of the magnetic field, which acts along 

 the coil axis. This position is shown at 6, Fig. 60. The vane in 

 seeking this position turns the spindle which carries the pointer. 

 The turning moment is opposed by springs. In the later models, 

 the coils of these instruments are surrounded by iron laminations 

 which shield them from stray fields. In the cheaper models, air 

 damping is used, being obtained by a light aluminum vane 

 attached to the moving element. The more expensive models 

 employ magnetic damping, such as is used with watthour meters, 

 a light aluminum vane moving between the poles of perma- 

 nent magnets. ' 



