this particular form of spring described in § 16, page 65. TT is a thin tube of charcoal iron, attached at its lower end to a brass cap c, terminated in a brass pin P, guided at the bottom in the way shown. To c is attached the lower end of the magnifying spring s (made of hard phosphor bronze), the upper end of which is attached rigidly to a brass pin p, passing through a hole in the glass top of the apparatus G G, and fastened by means of a screw and nut to the brass milled head H outside the glass top. This pin p, to which the upper end of the spring is attached, also serves as a guide for the top of the iron tube.

In the space w w there is wound a coil of insulated copper wire or a coil of copper strip, seen in section in the figure, the copper strip being insulated by a strip of paraffined silk being wound between the layers of copper. The ends of the coil are attached to the terminals. Now when a current is passed through this coil the iron tube is sucked down into it, and its lower end c, to which the spring is attached, receives a large rotatory motion, which is communicated directly to the pointer rigidly attached to the top of the iron tube. Parallax, in taking readings of the pointer, is avoided by the hori zontal scale having a piece of looking-glass let in it in the well-known way.

The equality of the divisions throughout the whole scale depends on the fulfilment of three conditions :

1st. Placing the iron tube just so far inside the coil that the downward pull for a given current varies but little with a small upward or downward position of the tube.

2nd. Employing the magnifying spring so that a large motion of the pointer is obtained with a very small downward motion of the iron tube.

3rd. Making the iron tube extremely thin so that it is "magnetically saturated" with a current passing round the coil smaller than the instrument is intended to measure. Hence, for all currents to be measured the

tube behaves as if it were a permanent magnet of constant strength.

Practically, then, the tube behaves as a permanent magnet in a fixed position. Hence, as explained in § 19, page 70, the force exerted is directly proportional to the current, and, since the rotation of the free end of a magnifying spring is directly proportional to the stretching force, it follows that the angular deflection of the pointer is directly proportional to the current. Practically, this is found to be true through a range of about 270°, excluding the first 15° corresponding with currents too small to saturate the iron, and therefore for the first 15° from zero the scale is not graduated.

Experiment shows that the iron tube should be placed with about two-thirds of its length inside the coil, for if an iron core be lowered into a coil round which a constant current is flowing the pull for a core and coil of the same length is found to increase until about two-thirds of the core is inside the coil, then it remains practically constant as the core is lowered through a certain distance, next the pull falls off with more or less rapidity, and finally becomes nought when the centre of the core coincides with the centre of the coil.

This instrument being direct-reading, has to be provided with an adjustment for sensibility, and this is obtained partly by the amount of wire or strip that is wound on the bobbin, and partly by means of a small movable bobbin, wound with a coil of fine wire of the same length as that employed in winding the main coil, joined up in parallel with the main coil. This movable coil slides up and down on the main bobbin, and by trial a position is found for it such that the readings on the dial are correct, and in that position this auxiliary coil is permanently fixed by the maker of the instrument.

The pointer will deflect in the same direction, no matter which way the current passes through the instrument, and owing to the softness of the iron used in

making the tube T T, and the smallness of its mass, there is but very little residual magnetism left in it; hence, the pointer indicates the correct strength of the current, no matter which way it passes through the instrument. To ascertain the direction of the current, a small compass needle is let into the base of the instrument, as seen in the front right-hand corner of Fig. 71, and, to avoid a reversal of the magnetism of this compass needle should a large current be suddenly sent through the ammeter, the little box containing the needle is made of fairly thick iron.

It might at first sight be thought that the indications of the magnifying spring ammeter would be easily affected by the presence of any neighbouring magnet. This, however, is not the case, and arises from the fact that the main motion of the iron tube is a bodily downward one, a motion of translation and not a motion of rotation. Now a motion of translation cannot be produced by a distant magnet, no matter how powerful it may be, whereas even a distant weak magnet can produce a rotatory motion and deflect the needle of an ordinary galvanometer. This experiment can be easily tried by floating a compass needle on a piece of cork in a basin of water and holding a magnet a foot or two away. The needle will turn so as to place itself along the lines of force, but if the magnet be far enough away the field is a uniform one over a small space round the needle (see § 19, page 70). The needle will not move bodily so as to approach the magnet no matter how strong the latter may be.

Hence, the magnetism of even a powerful dynamo machine two or three feet away from a magnifying spring ammeter will not affect its indications.

SOME MODERN FORMS OF AMMETERS.

37. Gravity Control Ammeters. Unless it is required to use an ammeter in several different positions, the control at the present day is usually produced by a

weight, and, in order that the same type of instrument may be easily modified during construction, so that the scale may either be approximately equally divided, or, instead, made very open at some particular part, the

deflecting force is often produced by the attraction or repulsion of two or more pieces of soft iron, one being fixed and the other movable, and both magnetised by the current flowing through a coil surrounding the entire soft-iron system.

Figs. 72, 73, and 74 represent the working parts of three such " gravity control ammeters," each shown two-thirds full size; the coil of wire and brass tube inside the coil being partially cut away so that the needle may be clearly seen. Figs. 72a, 73a, and 74a show the needles, also two-thirds full size, withdrawn from the brass tube. In Figs. 72, 73, 74, and 74a the needles are seen in the positions they occupy when no current passes round the coil, while in 72a and 73a they are shown as if deflected by a current passing round the coil strong enough to turn the pointer into a vertical position.

The first of this group of figures, Fig. 72, illustrates the type of gravity control ammeter constructed by Messrs. Schuckert at Nürnberg, and much employed on the Continent. The needle is made out of a sheet of thin soft iron bent so that it has one plane face A B C D (Fig. 72a), passing through the axis of rotation A D, and a curved face, B E. The weight of the pointer pp, which is attached to the needle along the edge A B of its plane face, A B C D, is adjusted so that when no current is passed round the coil the needle and pointer hang as is shown in Fig. 72, and also by the continuous white line in Fig. 726. If the axis of rotation AD were placed along the axis of the cylindrical coil, no deflection of the needle would be produced by any current passing round the coil, therefore the needle is pivoted eccentrically. And, since the magnetic field produced by a given current flowing round the coil increases in strength as we proceed from the axis of the coil outwards, the curved face of the needle is attracted into the position shown in Fig. 72a when the current is about half as strong as the ammeter is intended to measure, and into the position indicated by the dotted white line in Fig. 726 for a much larger

current.

The controlling couple is produced by the weight of the needle and pointer, the weight of the pointer assisting