40-4 



USE OF THE GALVANOMETER 



abscissas, and those of sin (n -f- m) as ordinates. 

 by taking sin 90°= 100, we should have — 



In the present case, 



The curve aR in the figure is drawn from these values. They show, 

 what is evident from a glance at the figure, that the culmination of 

 the curve is not over the zero point, but over a point between the 

 abscissas of 10° and 15°, and that it declines from this toward zero. 

 If drawn out completely, i. e.. continued on the other side of the 

 meridian to the abscissa of 90°, the curve would therefore have two 

 maxima. 



This circumstance, which, so far as I know, has not hitherto been 

 observed, is not caused by an error of observation ; I have satisfied 

 myself of its correctness by repeated measurements. Nor can it be 

 merely a peculiarity of my galvanometer, for it evidently arises from 

 the space which is left between the coils for the purpose of intro- 

 ducing the needle into the interior of the spiral. All galvanometers 

 which have such a space must exert an influence on the needle which 

 will be represented by a curve with two maxima, and few are with- 

 out this space.* It is, however, no disadvantage, provided the de- 

 pression be not too great ;t only, in consequence, the deflections will 

 not be proportional to the force of the current, within the first ten 

 degrees, as is usually assumed, but will bear a rather complicated 

 relation to it. This is, however, a matter of indifference, for by the 



° It has already been observed by several physicists that every copper wire, even that 

 which contains the least possible quantity of iron, is slightly magnetic, and therefore 

 renders it impossible to place a really astatic needle parallel to the coils of the wire, when 

 there is a space between the coils which offers two points of attraction. This observation 

 induced Peclet, (Ann. de Chim. et de Phys. ser. Ill, t. II, p. 103,) to fill up this space and 

 to hang the needle on a stirrup embracing the spiral. This arrangement would not show 

 the above-mentioned depression, hut it has other disadvantages, e. g. , it lessens the ampli- 

 tude of the needle very much, so that thtre is little probability of its general adoption. 



■j- If the depression were too great, it might happen that at one point the tangent of the 

 curve would form a greater angle with the line of abscissas than the tangent of the mag- 

 netic curve at the intersection of the two curves. The consequence of this would be, that 

 this point would correspond to an instable equilibrium between the galvanic and magnetic 

 forces, and there would be another point of intersection on each side of it, corresponding 

 to a stable equilibrium. The existence of these three points of intersection of the curves 

 a R and M N would make the empirical determination of the corresponding values of m and 

 n very complicated. The space which is usually left between the coils of the galvanometer 

 3, however, not great enough to cause so deep a depression of a R. 



