CHAP, in.] ELECTRICITY AND MAGNETISM. 153 



retreats ; while the current flowing above, which also 

 deflects the N. -seeking pole to its left, will equally urge 

 a towards the observer, and b from him. The needle 

 will not stand out completely at right -angles to the 

 direction of the wire conductor, but will take an oblique 

 position. The directive forces of the earth's magnetism 

 are tending to make the needle point north-and-south. 

 The electric current is acting on the needle, tending 

 to make it set itself west -and -east. The resultant 

 force will be in an oblique direction between these, 

 and will depend upon the relative strength of the two 

 conflicting forces. If the current is very strong the 

 needle will turn widely round ; but could only turn com- 

 pletely to a right-angle if the current were infinitely strong. 

 If, however, the current is feeble in comparison with the 

 directive magnetic force, the needle will turn very little. 



188. This arrangement will, therefore, serve roughly 

 as a Galvanoscope or indicator of currents ; for the 

 movement of the needle shows the direction of the 

 current, and indicates whether it is a strong or a weak 

 one. This apparatus is too rough to detect very delicate 

 currents. To obtain a more sensitive instrument there 

 are two possible courses : (/'.) Increase the effective 

 action of the current by carrying the wire more than 

 once round the needle : (#.) Decrease the opposing 

 directive force of the earth's magnetism by some com- 

 pensating contrivance. 



189. Schweigger's Multiplier. The first of the 

 above suggestions was carried out by Schweigger, who 

 constructed a multiplier of many turns of wire. A suit- 

 able frame of wood, brass, or ebonite, is prepared to 

 receive the wire, which must be " insulated," or covered 

 with silk, or cotton, or guttapercha, to prevent the 

 separate turns of the coil from coming into contact with 

 each other. Within this frame, which may be circular, 

 elliptical, or more usually rectangular, as in Fig. 80, the 

 needle is suspended, the frame being placed so that the 



