642 



THE MULTIPLICATOR. 



m 



and occupy different positions with reference to the needle. The multiplicator 

 is thus an apparatus by means of which a feeble current can readily be detected. 

 Experience has taught further that if the feeble galvanic current to be ex- 

 amined encounters great resistance in the closed circuit, such as in animal tissues 

 through which the current is passed, then many turns of a thin wire are to be 

 made about the needle. If, however, the conduction-resistance in the circuit is 

 but slight, as is the case, for example, in the application of the thermoelectric 

 apparatus, only a few turns of a thick conducting wire are made about the mag- 

 netic needle. 



In order to render the multiplicator more sensitive in another manner the 

 magnetic power of direction of the needle, by means of which it tends to turn 

 toward the north, can be enfeebled. The extent to which this has been attained 

 in the thermoelectrogalvanometer for the examination of feeble currents has 

 been described and illustrated in connection with the study of feeble thermic 

 currents (p. 333). It should be especially mentioned at this point that for the 

 demonstration of electrical currents in animal tissues a coil consisting of a large 

 number of turns of thin wire is to be attached to that instrument. 



In the multiplicator of Schweig- 

 ger, employed for physiological pur- 

 poses, the tendency of the needle to 

 point toward the north has been 

 materially enfeebled by the employ- 

 ment of the astatic pair of needles, 

 as suggested by Nobili. Two identi- 

 cal magnetic needles are attached 

 parallel one above the other by 

 means of a fixed middle piece of 

 horn, but in such a manner that the 

 north poles point in opposite direc- 

 tions. As it is impossible to impart 

 to each needle a magnetic strength 

 of absolutely equal degree, one of 

 the needles will thus be always some- 

 what stronger than the other. This 

 difference in strength should, how- 

 ever, not be so great that the stronger 

 needle is directed toward the north, 

 but it should be sufficient only to 

 cause the freely suspended pair of 

 needles to assume a certain angle to 

 the magnetic meridian, to which posi- 

 tion it always returns after having 

 been deflected therefrom, with the 

 execution of a number of progres- 

 sively diminishing oscillations. The 

 angle assumed by the astatic pair of 

 needles to the magnetic meridian is 

 designated the jree de-flection. The 

 greater the degree of astasia attained, 

 the more nearly will the angle formed 

 by the direction of the free deflection with the magnetic meridian approximate a right 

 angle. The greater the degree of astasia, the fewer will be the number of oscillations 

 made by the pair of needles in a given time, when they attempt, after deflection, 

 to resume their original position. The duration of each of these periodic oscilla- 

 tions will then be quite long. 



The multiplicator is so constructed that the direction of the needles is the same 

 as that of the coils of wire. The upper needle oscillates above a scale graduated 

 in degrees on which the extent of the deflection of the needle can be read. Even 

 the purest copper wire in the coil always contains a certain admixture of iron, 

 which exerts an attraction upon the magnetic needle. Therefore, a small fixed 

 magnetic rod, designated a correcting rod or compensatory magnet (r), is attached 

 to the multiplicator. This is directed toward the one pole of the upper needle and 

 diminishes the strength of the astatic needles to such a degree that the attracting 

 force in the coils of wire (in consequence of the iron present) is rendered ineffective 

 with respect to the force of the earth's magnetism. 



ffl 



FIG. 225. I, Diagrammatic representation of the multipli- 

 cator adjusted for the investigation of a muscle-current; 

 N NI, a static pair of needles suspended from a silk 

 thread G; P P, the conducting vessels with the muscle 

 M; II and III, other adjustments of the muscle; IV, 

 non-polarizable electrodes. 



