VOLTAIC ELECTRICITY. 247 



of the earth ; in the galvanometer this is obviated almost 

 wholly by the use of an astatic needle. In Oersted's 

 apparatus, the current makes but a single course about 

 the needle; in the galvanometer, the covered wire is 

 coiled many times about the needle and thus the effect is 

 multiplied. Cue of the needles is within the coil while the 

 other swings above it, the two being connected by a 

 vertical axis passing through an appropriate slit in the coil. 

 If both needles were within the coil, since their poles are 

 reversed ( 314), the same current would tend to deflect 

 them in opposite directions and thus the action of one needle 

 would neutralize that of the other. The astatic needle is 

 suspended by an untwisted silk fibre from a hook which 

 may be lowered when the instrument is not in use until the 

 upper needle rests upon the dial plate beneath it. The ends 

 of the coiled wire are connected with bin ding screws; level- 

 ling screws are provided, by means of which the instrument 

 may be adjusted so that the needles shall swing clear of all 

 obstructions. A glass cover protects from dust and dis- 

 turbance by air currents. The instrument is represented 

 in Fig. 192. 



392. Magnetic Effects of the Voltaic Cur- 

 rent. A bar of soft iron may be easily 

 magnetized by the inductive influence of 

 the Voltaic current. This is shown by 

 the action of the bar and helix (Fig. 193). 



(a.) Tliis apparatus consists of a movable bar FIG. 



of soft iron surrounded by a coil of insulated 

 copper wire. When the wire of the coil is placed in the closed 

 circuit of a battery, the iron bar becomes strongly magnetized ; 

 when the circuit is broken, the bar instantly loses its magnetic 

 power. The same thing is illustrated by the ' ' helix and ring 



