544 



GALVANOMETER. 



by coiling the conducting wire many times in the same direction on a rectangular frame, 

 or merely around and in the same direction as the needle, [provided that each turn 

 of the wire be properly insulated from the other]. An instrument constructed on this 

 principle is called a multiplicator or multiplier. The greater the number of turns of the 

 wire, the greater is the augle of deflection of the needle, although the deflection is not directly 

 proportional, as the several turns or coils are not at the same distance from, or in the same 

 position as, the needle. By means of the multiplier we may detect the presence [and also the 

 amount and direction] of feeble currents. [The instrument is now termed a galvanometer.] 

 Experience has shown that, when great resistance, (as in animal tissues), is opposed to the weak 

 galvanic currents, we must use a very large number of turns of thin wire round the needle. If, 

 however, the resistance in the circuit is but small, e.g., in thermo-electrical arrangements, a few 

 turns of a thick- wire round the needle are sufficient. The multiplier may be made more sensitive 

 by weakening the magnetic directive force of the needle, which keeps it pointing to the north. 



Galvanometer and Astatic Needles. In the multiplier of Schweigger, used for physiological 

 purposes, the tendency of the needle to point to the north is greatly weakened by using the 



astatic needles of Nobili. [A multiplier 

 or galvanometer with a single magnetic 

 needle always requires comparatively 

 strong currents to deflect the needle. 

 The needle is continually acted upon by 

 the directive magnetic influence of the 

 earth, which tends to keep it in the 

 magnetic meridian, and, as soon as it is 

 moved out of the magnetic meridian, 

 the directive action of the earth tends 

 to bring it back. Hence, such a simple 

 form of galvanometer is not sufficiently 

 sensitive for detecting feeble currents. 

 In 1827, Nobili devised an astatic com- 

 bination of needles, whereby the action 

 of the earth's magnetism was dimin- 

 ished.] Two similar magnetic needles 

 are united by a solid light piece of horn 

 [or tortoise shell], and are so arranged 

 that the north pole of the one is placed 

 over or opposite to the south pole of the 

 other (fig. 380). [If both needles aro 

 equally magnetised, then the earth's 

 influence on the needle is neutralised, 

 so that the needles no longer adjust 

 themselves in the magnetic meridian ; 

 hence, such a system is called astatic] 

 As it is impossible to make both needles 

 of absolutely equal magnetic strength, 

 one needle is always stronger than the 

 other. The difference, however, must 

 not be so great that the stronger needle 

 points to the north, but only that the 

 freely suspended system of needles forms 

 a certain angle with the magnetic me- 

 ridian, into which position the system 

 always swings after it is deflected from 



astatic needles 

 P, non-polaris- 



Fig. 380. 

 Scheme of the galvanometer. X. 

 suspended by the silk fibre, G ; 

 able electrodes, containing zinc sulphate solution, 

 s, and pads of blotting paper, b, covered with clay, 

 t, t, on which the muscle, M, is placed ; II and III, 

 arrangement of the muscle on the electrodes ; IV, 

 non-polarisable electrodes ; Z, zinc wire ; K, cork ; 

 a, zinc sulphate solution ; t, t, clay points. 



this position. This angular deviation of the astatic system towards the magnetic meridian is 

 called the " free deviation." The more perfectly an astatic condition is reached, the nearer does 

 the angle formed by the direction of the free deviation with the magnetic meridian become a 

 right angle. The greater, therefore, the astatic condition, the fewer vibrations will the astatic 

 system make in a given time, after it has been deflected from its position. The duration of 

 each single vibration is also very great. [Hence, when using a galvanometer, and adjusting its 

 needle to zero, if the magnets dance about or move quickly, then the system is not sensitive, 

 but a sensitive condition of the needles is indicated by a slow period of oscillation.] 



In making a galvanometer, the turns of the wire must have the same direction as the needles. 

 In Nobili's galvanometer, as improved by du Bois-Reymond, the upper needle swings above a 

 card divided into degrees (fig. 380), on which the extent of its deflection may be read off. Even 

 the purest copper wire used for the coils round the needles always contains a trace of iron, which 

 exerts an influence upon the needles. Hence, a small fixed directive or compensatory magnet 

 (r) is placed near one of the poles of the upper needle to compensate for the action of the iron 

 on the needles. 



