GALVANOMETEK. 



winding the wire which carries the current several times round 

 the needle, each coil being still parallel to the needle. By this 

 contrivance, each successive coil of the wire produces a separate 

 effect upon the needle, and if there he fifty such coils passing suc- 

 cessively before and behind the needle, each portion of the wire 

 thus carrying the current producing an independent deflecting 

 force, there will be a total deflecting force an hundred times greater 

 than that which a single portion of the wire, passing once over or 

 under the needle would produce. 



In this manner the deflecting power of the most feeble current 

 may be so multiplied as to produce upon the needle as powerful an 

 effect as would be produced by a current of great intensity. 



An apparatus consisting of wire thus coiled round a magnetic 

 needle is called a ITTJLTIPLIEE, inasmuch as it multiplies the 

 deflecting power of the needle. It is also called a EEOSCOPE, or 

 BEOMETEE,* and sometimes a GALVANOSCOPE, or GALVAXOMETEE, 

 inasmuch as it indicates the presence, and by certain arrange- 

 ments, measures the intensity of a galvanic or voltaic current. 



139. When the conducting wire is thus coiled round a needle, 

 it is necessary that it should be covered or coated by some substance 

 which is a non-conductor of electricity, since otherwise the coils 

 being necessarily in contact one with another, the current, instead 

 of following the continuous thread of wire, would pass from coil to 

 coil. In such cases, therefore, the wire is wrapped with silk or 

 cotton, which being a non-conductor, confines the current within 

 it just as water would be included in a pipe. 



140. As the wire coiled in the manner above-described, and the 

 frame which carries it, would prevent the play of the needle from 

 being easily and conveniently observed, the needle included within 

 the frame is fixed upon the axis which supports it, so that the 

 axis turns with it. This axis passes through the side of the frame, 

 on which the wire is coiled, and upon the end of it which projects 

 beyond the frame a hand is fixed, so as to be parallel to the needle, 

 the play of which will necessarily correspond with that of the 

 needle. This hand plays upon a sort of dial, by which its devia- 

 tions to the right or to the left, from its position of rest are 

 indicated. 



This will be more clearly understood by reference to fig. 55 

 (p. 196), which represents a section of the mounting of the needle, 

 the coil of wire and their appendages, made by a vertical plane 

 through the axis of the needle. The needle within the coil is 

 represented at a b, in its position of rest. The axis of the needle 



* From two Greek words, peos (reos) a current, and nerpov (metron) a 

 measure. 



2 195 



