A light rectangular coil of copper wire, C, is wound on an aluminium frame pivoted in jeweled bearings so as to be free to rotate in the ring-like space between an inner cylindrical soft iron core, K, and the pole pieces P and P of the permanent magnet, M. A light aluminium pointer, p, is attached to the coil and is free to move across the scale, D. The current enters the coil through the two spiral springs S and S, which serve also to control the movement of the coil. When a current passes through the coil the dynamic action between the current and the magnetic field tends to rotate the coil, and the position of equilibrium between this force and the torsion of the springs, indicated by the pointer, measures the current passing through the coil. Because the magnetic field is practically unvarying throughout, and the torsion of the springs is proportionate to their deflection, the scale is virtually uniform. This is not assumed in their manufacture, however, for each instrument is calibrated by direct reference to standards. As the aluminium frame moves through the magnetic field, slight currents are generated within the metal; these serve to dampen vibrations so that the pointer comes to rest almost instantly without friction. That the magnetic field may have the utmost strength, the air gap in which the coil rotates is made as narrow as possible; this is ensured by workmanship of the highest skill, and by tools specially designed. The hardened steel pivots are ground and centered as in the best watch-making: the coil is balanced by means of adjustable weights so that none but electrical forces may come into play. In a Weston voltmeter of regular type, the maximum current required for a full scale-deflection is only 0.01 ampere. Instruments of much higher sensibility are constructed for measuring insulation, requiring but 0.0006 ampere for the same deflection. So much for the task of measuring electrical pressure. For measuring electrical currents, which differ from pressures as the quantity of water flowing in a pipe differs from the pressure of that water as shown in a common gauge, a Weston ammeter, or ampere-meter, may be employed. It is similar to the voltmeter just described, being in fact a milli-voltmeter actuated by the difference in electrical potential, or pressure, between the terminals of a standard resistance, the shunt, through which a definite fraction of the current passes. It is as if a known part of the flow of a river being measured, the volume of the whole stream is learned.