The first-named instruments are based on the principle, set forth in Chap. III. above, of the mutual action between the fixed and movable portions of a circuit carrying a current. Each of the mutually influencing portions consists in most of the instruments of one or more complete turns or spires of the conductor, but in some cases consists of only half or part of a turn. In all cases in what follows we shall, following Sir W. Thomson, call each portion a ring.

In each of the balances, except that for very strong currents (the kilo-ampere balance), the movable portion of the conductor consists of two rings, carried with their planes horizontal at the extremities of a balance beam free to turn in the ordinary way round a horizontal axis. Above and below each ring on the beam is a fixed ring with its plane parallel to that of the movable ring. The rings are (except in the Composite Balance, p. 107 below) all joined in series, and the current to be measured is sent through them so that the mutual action between the movable ring at one end and each of the two fixed rings there is to raise that movable ring, while the mutual action of the other group of three rings is to depress the corresponding movable ring. The action is therefore to turn the beam round the horizontal axis on which it is pivoted, with for any given position a couple varying as the square of the current flowing.

Fig. 11 shows diagrammatically the rings and the course of the current through them. a, e, b, ƒ are the two pairs of fixed rings, c, d the movable rings. The current en

tering by the terminal T passes round all the rings in series, in the two movable rings in opposite directions, and returns to the terminal T. Since each movable ring is in general in a magnetic field, terrestrial or

artificial, which has a horizontal component, it tends to set itself so that the greatest number of horizontal lines of force may pass through it (Chap. III. above) and therefore is acted on by a couple which tends to turn the beam round its axis. But since the current passes round the movable coils in opposite directions, and these are very approximately equal, the two couples are nearly equal and opposite, and the instrument is practically free from disturbance by horizontal magnetic force.

a

C

T

FIG. II.

The turning couple produced by the mutual action of the fixed and movable rings is balanced for the horizontal or "sighted position" of the beam by an equal and opposite couple produced (in the manner more particularly described below) by a stationary weight at the end of the beam, and a sliding weight placed, steelyard fashion, at a suitable point on a graduated bar attached to the beam. The amount of the current flowing in the rings is de

H

duced from the amount of the equilibrating couple thus applied, or rather from a number proportional to it, by the end of a table of reckoning, as described below.

In the kilo-ampere Balance (Figs. 12 and 15) there are only two rings, each nearly rectangular in shape, and horizontal in position. These, since they have to bear very strong currents, are made of massive copper strip. The

FIG. 12.

movable ring is mounted as shown above the fixed ring, and is free to turn round a horizontal axis halfway between its two ends. In the diagram (Fig. 12) a plan of the fixed and movable rings (A, B, respectively), with the course of the current in them, is shown. The current enters the movable ring by the terminal T, and divides, passing round its two halves in opposite directions. At

the middle of the other side the two parts of the current meet and pass to the fixed ring, then completely round that ring once or more times, and across to the terminal T. One half of the movable ring thus tends to rise, the other to fall, and hence the ring tends to turn on its axis. The turning couple is resisted by a return couple due to weights precisely as in the former case.

Having thus stated the general principle of these instruments, we now consider their construction and mode of action with greater minuteness. Most of the constructive details will be made out from Fig. 13 which shows what is called the Standard Centi-ampere Balance, and illustrates the arrangement of the beam, the graduation, and the mode of applying the equilibrating couple, for all the instruments.

The beam in all the instruments is hung on two trunnions, each supported by a flat elastic ligament made of fine copper wires, through which the current passes to and from the movable rings.

i The horizontal or sighted position of the beam is that in which the pointers on the extreme right and left are at the middle divisions of their scales. This position, in all the instruments in which a movable ring is acted on by two fixed rings between which it is placed, is not that midway between these two rings, as that would be a position of minimum force and therefore of instability. For stability it is so chosen that the movable ring is nearer to the repelling fixed ring than to the attracting ring by such an amount as to give about per cent. more than the minimum force.

Fixed to the beam and parallel to it is a finely graduated bar, and above this is a horizontal fixed scale, called the Inspectional Scale, less finely divided. Both gradua