272 W. WEBER ON THE ARRANGEMENT AND 



Even in the construction of the Unifilar Magnetometer it was 

 noticed that it would be convenient that the torsion circle, of 

 which frequent use is made, should be fixed to the stu-rup of the 

 magnetometer instead of to the ceihng. The same object has 

 been considered in the construction of the bifilar, where, on 

 account of the greater height of suspension, it was of much 

 more consequence. To make it quite unnecessary to go to the 

 ceiling in the case of the bifilar, several other aiTangements are 

 requisite at the stiiTup. The screws, for instance, which serve 

 to lengthen and shorten the wires, have to be fixed to the stirrup 

 instead of the ceiling. They are very clearly marked in PI. XIV., 

 figs. 1, 2, 3, where it is seen how they are connected with the 

 circle on which the stirrup is placed, and arranged in the same 

 way as the elevating screw of the unifilar, so that the wires may 

 be lengthened or shortened without any lateral displacement. 



It is also necessary to be able to bring the two wires closer to, 

 or further from each other, at the stirrup, so as to increase or 

 diminish their directive force at pleasure. Although it is most 

 simple that the two wires which support the instrument should 

 be always equi-distant above and below, and that whenever it 



we resolve this force into a force acting in its magnetic axis, by multiplying 



^ D 

 the entire force by the fraction —j-r,> and into a force perpendicular to it (to- 

 wards the magnetic meridian) by multiplying the entire force by the fraction 



D P 



-„ we obtain for i\\(i first the value 



AD AF^ Min__AB^ Mm _ M m 

 ATF • (TF ■ ZC3~ CF • .'/ C's— A C'3" '^ ' 

 and for the latter the value 



DF AF Mm __ DF Mm _ Mm 

 A F ■ ITF ' AC^~ CF ' AC'- AC^' 



smce2CF=CD = AD ^2, or ^^= ^ 2, and CF=DF. 

 The force -j-^ V^ directly changes the directive force of the intensity-bar 



in its transversal position. This force -jy^ would alter its position if the effect 



were not counteracted by asuitable'change in the suspension, sothatthebar should 

 remain unmoved in its transversal position. In the latter case it is true that 



the force "' 3 no longer conies into consideration ; but the changed suspension 



has certainly some influence on the directive force, and consequently on the 

 value of the"divisi( ns of the scale. This, however, does not require any separate 

 calculation, being included in the calculation of the directive force from the 

 given suspension, — a problem which belongs to i\\Q Theory of the Bifilar Mag- 

 netometer, which will subsequently be developed. 



