obtained, when the armature is rotating. The proper position for the needle, _b_, is that in which it is just leaving the mercury, as the keeper arrives at the position, in which its magnetism is neutralized. This position is seen at X, where D″ and D′ are the sides of the coils; _c_ the copper disc; _m_ the cross bar of the armature; R the arm passing through the coil; and _b_ the needle, at that _angle_ which it requires, when the armature is vertical or at its neutral position. It will be observed that the needle is just leaving the mercury, _d_. [Illustration: FIG. 44.] Figure 44 represents a top view. N and S represent the north and south poles of the permanent magnet. N′ and N′ is the spindle, parallel with the prongs of the magnet, and equidistant from them; L is the socket of the band wheel; D′ and D the horizontal position of the coils; M is the cross bar; _b_ the needle; _c_ the copper disc, and _m_ and _n_ their respective sockets; _o_ the ivory hub; _d_ the cup of mercury; A the platform; and S′ and S′ the yoke through which pass two screws to secure the magnet to the wooden support below. When the armature is made to rotate, it becomes a temporary magnet, by the laws of magnetic induction, whenever the arms carrying the helices come opposite to the poles of the permanent magnet, and when these soft iron arms have reached the point at _right angles_ to the _magnet_, or vertical, their magnetism for an instant is destroyed, and are as instantaneously reversed from what they were before reaching that point. They are also magnetic, just in that proportion as they recede from or approach to the poles of the permanent magnet. Hence, first, _one arm_ is the south pole, when opposed to the north pole of the magnet; and the _other arm_ a north pole, when opposed to the south pole of the magnet. But when they have made a half revolution on their axis, from their first position, their magnetism is reversed. The arm which was a south pole, has become a north pole; and the arm that was a north pole has become a south pole. Thus, by the rotation of the armature, direction of the induced current in the arms, become changed, as often as they are alternately brought opposite the poles of the permanent magnet, which is _twice_ in every revolution of the armature.