KHEOOORD GALVANOMETER. 



543 



tricity must be of greater density in the narrower parts, and it is evident that the density will 

 be less where the transverse section is greater. Let S = the strength of the current, and q the 

 transverse section of the given part of the circuit, then the density (d) at the latter part is 



If the galvanic current passing from the positive pole of a battery is divided into two or 

 more streams, which are again reunited at the other pole, then the sum of the strength of all 

 the streams is equal to the strength of the undivided stream. If, however, the different streams 

 are different as regards length, section, and material, then the strength of the current passing 

 in each of the streams is inversely proportional to the resistance to the conduction. 



Du Bois-Reymond's Rheocord. This instrument, constructed on the principle of the 

 "secondary" or "short circuit," enables us to graduate the strength of a galvanic current to 

 any required degree, for the stimulation of nerve and muscle. 

 From the two poles (fig. 379, a, b) of a constant battery, there 

 are two conducting wires (a, c and d, b), which go to the 

 nerve of a frog's nerve-muscle preparation (F). The portion 

 of nerve (c, d) introduced into this circuit {a, c, d, b) offers 

 very great resistance. The second stream or secondary circuit ' 

 (aA,JB) conducted from a and b passes through a thick 

 brass plate (A, B), consisting of seven pieces of brass (1 to 7) 

 placed end to end, but not in contact. They can all, with 

 the exception of 1 and 2, be made to form a continuous 

 conductor by placing in the spaces between them the brass 

 plugs (Sj to S 5 ). Evidently, with the arrangement shown 

 in fig. 379, only a minimal part of the current will pass 

 through the nerve (c, d), owing to the very great resistance 

 in it, while by far the greatest part will pass through the 

 good conducting medium of brass (A, L, B). If new resist- 

 ance be introduced into this circuit, then the a, c, d, b 

 stream will be strengthened." This resistance can be intro- 

 duced into the latter circuit by means of the thin wires 

 marked I a, I b, I c, II, V, X. Suppose all the brass plugs 

 from S x to S 5 to be removed, then the current entering at 

 A must traverse the whole system of thin wires. Thus, 

 there is more resistance to the passage of this current, so 

 that the current through the nerve must be strengthened. 

 If only one brass plug be taken out, then the current passes 

 through only the corresponding length of wire. The resist- 

 ances offered by the different lengths of wire from I a to 

 X are so arranged that I a, I b, and I c each represents a 

 unit of resistance ; II, double ; V, five times ; and X, ten 

 times the resistance. The length of wire, I a, can also be 

 shortened by the movable bridge (L) [composed of a small 

 tube filled with mercury, through which the wires pass], 

 the scale (x, y) indicating the length of the resistance wires. 

 It is evident that, by means of the bridge, and by the 

 method of using the brass plugs, the apparatus can be 

 graduated to yield very variable currents for stimulating 

 nerve or muscle. When the bridge (L) is pushed hard up to 1 and 2, the current passes 

 directly from A to B, and not through the thin wires (I a). 



The rheostat is another instrument used to vary the resistance of a galvanic current 

 ( Wheatstone). 



327. ACTION OF THE GALVANIC CURRENT ON A MAGNETIC NEEDLE GAL- 

 VANOMETER. In 1820, Oerstedt of Copenhagen found that a magnetic needle, suspended in 

 the magnetic meridian, was deflected by a constant current of electricity passed along a wire 

 parallel to it. [The side to which the north pole is deflected depends upon the direction of the 

 current, and whether it passes above or below the needle.] 



Ampere's Rule. Ampere has given a simple rule for determining the direction. If an 

 observer be placed parallel to and facing the needle, and if the current be passing from his feet 

 to his head, then the north pole of the needle will always be deflected to the left, and the south 

 pole in the opposite direction. The effect exerted by the constant current acts always in a 

 direction towards the so-called electro-magnetic plane. The latter is the plane passing through 

 the north pole of the needle, and two points in the straight wire running parallel with the 

 needle. The force of the constant current, which causes the deflection of the magnetic needle, 

 is proportional to the sine of the angle between the electro-magnetic plane and the plane of 

 vibration of the needle. 



Multiplicator. The deflection of the needle caused by the constant current may be increased 



Fig. 379. 



Scheme of du Bois-Reymond';- 



rheocord. 



