THE MULTIPLICATOR. 



641 



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this arrangement, as depicted in Fig. 224, only a minimal current passes through 

 the segment of nerve (c d), which offers great resistance, while by far the greater 

 portion of the galvanic current passes through 

 the well-conducting brass plate (A - B) . If in- 

 creased resistance be introduced into this latter 

 circuit, the branch current ac d b must naturally 

 be increased correspondingly. These resistances 

 can be interposed by means of the portions of 

 fine wire indicated by the letters la, I b, I c, II, 

 V, X. If it be supposed that all of the brass 

 plugs (from S t to S 5 ) are withdrawn, the branch 

 current entering at A must pass through the en- 

 tire system of fine wire. In this way a high de- 

 gree of resistance is interposed and the branch 

 current in the nerve must be increased corre- 

 spondingly. If but one plug is withdrawn, the 

 current passes only through the respective length 

 of wire. The resistances offered by the various 

 lengths of wire (from I a to X) are so related 

 that la, Ib, and Ic each represents a unit of con- 

 duction-resistance, II twice as much, V five times 

 as much, and X ten times as much resistance. 

 The distance I a may finally be lessened by the 

 bridge (L), which can be moved upward, the 

 scale (x y) indicating the length of the resistance- 

 distance. It will be readily perceived that in 

 accordance with the manner of applying the 

 plugs and the bridge, the apparatus permits of a 

 varied gradation in the branch current to be 

 sent through the nerve. If the bridge L is 

 pushed up close to i, 2, the current passes 

 directly from A to B, and not through the 

 length of thin wire I a. 



Other forms of apparatus intended for intro- 

 duction into the closing arc of a circuit, in order 

 to increase the conduction-resistance at will, are FlG ' f 2 thT 

 designated rheostats. mond 6 



THE ACTION OF THE GALVANIC CURRENT UPON THE MAGNETIC 

 NEEDLE. THE MULTIPLICATOR. 



If a galvanic current be passed (for example through a wire) parallel to a 

 magnetic needle, the latter will be deflected from its position pointing to the 

 north. If it be conceived that one is swimming in the positive current, the head 

 in front and the abdominal surface directed toward the needle, the north pole 

 of the magnetic needle will always be deflected toward the left (Ampere's rule). 

 The deflecting force exerted by the galvanic current upon the needle always 

 operates at right angles to the so-called electromagnetic plane, that is the plane 

 passing through the north pole of the needle and two points in the conducting 

 wire (passing in a straight direction parallel to the needle). If, for example, 

 the conducting wire passes just above and parallel with the magnetic needle, 

 whose plane of oscillation is formed by the horizontal surface, the electromagnetic 

 plane will be vertical to the horizontal plane, and it will pass through the north 

 pole of the needle and the conducting wire. The strength of the galvanic current 

 that causes the deflection of the magnetic needle is proportional to the sine of 

 the angle between the electromagnetic plane and the plane of oscillation of the 

 needle. 



This deflecting power of the galvanic current can be increased if the con- 

 ducting wire is passed, instead of once, several times in the same direction in 

 front of the magnetic needle. An apparatus constructed according to this princi- 

 ple is designated a multiplicator. In this the conducting wire passes in numerous 

 turns at right angles to the horizontal plane around the magnetic needle sus- 

 pended in the middle and swinging in the horizontal plane. The larger the number 

 of turns the greater will be the angle of deflection of the needle, although not 

 exactly in direct proportion, as the individual turns are at varying distances 



