the Construction of Galvanometers, ^x. 245 



needle being adjusted, the spindle is to be disengaged from the forceps. If the 

 reversed poles precisely coincide in the same vertical plane, the needle will 

 oscillate, and finally settle in the magnetic meridian, provided that it retains 

 sufficient directive tendency. If the needle do not lie directly north and south, 

 other trials must be made, and generally the process of perfect adjustment is 

 tedious and troublesome. 



It sometimes happens, as already observed, that the magnetism of the bars 

 is equal, and the needle has no directive power. In such a case, the shghtest 

 touch of a magnetized wire to any one of the poles of the compound needle 

 will increase or lessen its power, and thus alter the balance: the slighter the al- 

 teration the better. 



The predominant magnetism should be so feeble, that the needle will very 

 slowly fall into the magnetic meridian ; but the predominance should be ade- 

 quate to produce that eflect. If the bars be not precisely in the same vertical 

 plane, they will have a weak tendency to cross the magnetic meridian : but 

 having also a directive power, they will be acted upon by the two forces, and 

 will point at the degi'ee on the graduated circles which expresses the balance of 

 forces. In this state the needle is unfit for service ; for although the defiec- 

 tion from the magnetic meridian thus produced may amount to a few degrees 

 only, it will resist a weak tendency to deflection in the opposite direction, when 

 the galvanometer is employed to measure voltaic action, and may modify even a 

 stronger one. Thus the condition of the needle most conducive to sensibility is 

 that of being retained precisely in the magnetic meridian, with the feeblest pre- 

 dominance of magnetism in one bar, which is adequate to this effect. 



A needle is frequently found to lie in the magnetic meridian, not because 

 its bars coincide with precision in the same vertical plane, for perhaps they do 

 not at the time, but because the magnetism of one or the other bar is so strong 

 as to overpower the great error which would have arisen in consequence. The 

 volta-magnetometer will detect the offending bar, by the inequality of the 

 angles to which the poles will point on the graduated circles when the bars are 

 opened out, and it may be weakened to the necessary degree by the similar 

 pole of the magnetized wire. Needles which point truly north and south, on 

 account of this inordinate predominance of power in one bar, are deficient in 

 sensibility ; and when a needle has extreme sensibility, it may be that it points 

 erroneously for want of sufficient predominance. 



2k2 



