154 ELEMENTARY LESSONS ON [CHAP. in. 



wires lie in the magnetic meridian. The greater the 



number of turns the more 

 powerful will be the mag- 

 netic deflection produced 

 by the passage of equal 

 quantities of current. But 

 if the wire is thin, or the 

 number of turns of wire 

 numerous, the resistance 

 thereby offered to the flow 

 of electricity may very 

 V* greatly reduce the strength 

 -P of the current. The student 



will grasp the importance 



of this observation when he has read the chapter on 

 Ohm's Law. 



19O. Astatic Combinations. The directive force 

 exercised by the earth's magnetism on a magnetic needle 

 may be reduced or obviated by one of two methods : 



(a.) By employing a compensating magnet. An ordinary 

 long bar magnet laid in the magnetic meridian, but with 

 its N.- seeking pole directed towards the north, will, if 

 placed horizontally above or below a suspended magnetic 

 needle, tend to make the needle set itself with its S. -seek- 

 ing pole northwards. If near the needle it may over- 

 power the directive force of the earth, and cause the 

 needle to reverse its usual position. If it is far away, all 

 it can do is to lessen the directive force of the earth. 

 At a certain distance the magnet will just compensate 

 this force, and the needle will be neutral. This arrange- 

 ment for reducing the earth's directive force is applied 

 in the reflecting galvanometer shown in Fig. %i, in 

 which the magnet at the top, curved in form and capable 

 of adjustment to any height, affords a means of adjust- 

 ing the instrument to the desired degree of sensitiveness 

 by raising or lowering it. 



(b.) By using an astatic pair of magnetic needles. 



