24' Electro-Magnetic Apparatus. 



to the magnetic meridian. W and E, are two pieces of card, with 

 letters on them, to show which side of the coil will turn to the east 

 or west : they may be properly placed by recollecting that the cur- 

 rent from the copper to the zinc has a tendency to circulate in a 

 direction contrary to that of the sun. 



Fig. 4, is designed to show the action of two conjunctive wires 

 on each other ; A B, is a thick multiplying coil, with galvanic 

 plates attached, in the same manner as shown in Fig. 2 ; c d, is a 

 lighter coil, with a double cylinder, precisely similar to Fig. 3, and 

 suspended within the other by a fibre of silk, passing through a 

 glass tube, (a) the end of which is inserted into an opening (b) in 

 the upper side of A B ; e f are two wires supporting the glass 

 tube. When the cylinder g and the plate C are placed in vessels 

 of .acidulated water, the inner coil will immediately arrange itself 

 so that the currents in both coils will circulate the same way : if the 

 vessel be removed from C, and D placed in the fluid, the coil c d 

 will turn half-way round and again settle, with the currents flowing 

 in the same direction. Instead of the cylinder, a separate battery 

 of [greater power may be used, by suspending the inner coil, as 

 shown in Fig. 9 ; h h are cups with mercury — the upper wire 

 should turn on a fine steel point. 



Fig's 5 and 6, are front and side views of a modification of an 

 instrument, described by Mr. Sturgeon. It consists of a dipping 

 needle, surrounded by a multiplying coil, turned edgewise, but in 

 all other respects similar to that of Fig. 1. If, when the needle is 

 placed in the magnetic meridian, and the coil in the plane of the 

 dip, a galvanic current be passed through it in a direction opposite 

 to that of the sun, the north end of the needle will turn up, as in 

 Fig. 7 ; but if in the contrary direction, it will turn down, as Fig. 

 8. If the coil be placed at right angles to the dip, as shown in the 

 dotted lines, and the current passed in the first mentioned direc- 

 tion, the needle will not alter its position, but will be more firmly 

 fixed in it : if passed in the contrary direction, it will turn half-way 

 round and dip with its south end. The quadrant q permits the 

 coil to be readily placed, either in the plane of the dip or at right 

 angles to it. 



