ON SOME MARVELS IN TELEGRAPHY, 243 



delicately poised, above or below which passed a wire con- 

 veying a galvanic current from a distant station. But the 

 effect can be intensified, as follows : 



FIG. i. 



Suppose abcdef\.Q\)Qa. part of the wire from A to B, 

 passing above a delicately poised magnetic needle N.S, along 

 a b and then below the needle along c d, and then above 

 again along e f, and so to the station B. Let a current 

 traverse the wire i n the direction shown by the arrows. Then 

 N, the north end of the needle, is deflected towards the east 

 by the current passing along a b. But it is also deflected 

 to the east by the current passing along c d; for this 

 produces a deflection the reverse of that which would be 

 produced by a current in the same direction above the 

 needle that is, in direction b a, and therefore the same as 

 that produced by the current along a b. The current along 

 e f also, of course, produces a deflection of the end N to- 

 wards the east. All three parts, then, a b, c d, e f, conspire 

 to increase the deflection of the end N towards the east. 

 If the wire were twisted once again round N s, the deflection 

 would be further increased ; and finally, if the wire be coiled 

 in the way shown in Fig. i, but with a great number of 

 coils, the deflection of the north end towards the east, 

 almost imperceptible without such coils, will become suffi- 

 ciently obvious. If the direction of the current be changed, 

 the end N will be correspondingly deflected towards the 

 west. 



The needle need not be suspended horizontally. If it 

 hang vertically, that is, turn freely on a horizontal axis, and 

 the coil be carried round it as above described, the deflec- 

 tion of the upper end will be to the right or to the left, 

 according to the direction of the current. The needle 

 actually seen, moreover, is not the one acted upon by the 



