MEASURING I NST It UM I.:NTS. I.:IA;(1'1{11`A L. It needs no demonstration to show that accurate riir the measurement. of electricity, especially when the sante is used as a source of power or of are of as much importance as accurate for measurement of steam. A which will not measure the expended within 5 or 10 per cent, is simply blind to losses of in the cost of power. Up to within a comparatively few years. accurate electrical did not exist outside of physical labora tories; and such instruments as were there employed were, front the very nature of their eon strut:Gum and the delicacy 1'0 I II red ill their unlit for the comparatively usage of the electric-lighting station. The need has been urgent for electrical gauges which are both simple and accurate—simple, in the sense that their mechanical parts should be few and easily adjusted ; accurate, in the sense that their operation should be certain, and the error so small as practically to be neglected.
A most important series of electrical measuring instruments, designed to meet these con ditions, has been invented within the last four years by Mr. Edward Weston. It is impos sible, within any space that can here be afforded, to describe all the runny forms of entirely novel instruments which Mr. Weston has produced, and of which it may safely be stated that they are rapidly revolutionizing modern methods of practically measuring the electric current. Two of the principal forms are, however, illustrated in Figs. 1 to 4.
The Weston Direct Current Volt and Anirne ter.—A perspective view of the exterior of this instrument is given in Fig. 1. The details of the mechanism will be clearly understood from Figs. 2 and 3. To the inner sides of the poles of a permanent magnet (Fig. 2) are secured cored-out pole-pieces. In the cylindrical space formed between these pole-pieces is supported a solid cylinder of magnetic material, by means of a brass bar bolted to the end of the mag net, and shown broken away in Fig. 2, This solid cylinder of magnetic material draws into
itself the lines of force from the magnet-poles, so that in the annular space between the cylinder and pole-pieces an exceedingly intense field of force is produced. Surrounding the fixed cylinder is a coil of fine insulated wire, shown separately in Fig. 3. This coil is pivoted in caps, which are supported on the pole-piece. Volute springs similar to those used in watches are fastened to the core-pivots and to fixed abutments, and operate to oppose any movement of the coil upon its pivots. The index-needle is also supported on the coil-pivot, so that it moves, as shown in Fig, 1, over the scale.
The foregoing is practically all there is in the mechanism of one of the most accurate in struments ever contrived—so accurate, indeed, that in ?lr. Weston's own laboratory it has displaced standard tangent gal vanometers of the most costly construction. The current to be measured is by suitable elec trical connections caused to trav erse the spiral springs and the coil entering one spring, going through the coil and coining, out at the other spring. When the coil is thus traversed by the cur rent, there is produced about it a field of force which reacts upon the permanent magnet field. The coil is therefore, in accord ance with well-known electrical laws, caused by the reaction of these two fields to turn on its pivots, and the extent of its an gular motion is always depend ent upon the difference of poten tial between the terminals of the instrument. If, then, the cur rent be directed through a com paratively high resistance ar ranged in series with the coil, the apparatus becomes adapted for use as a voltmeter, or for measuring electrical pressure, and the scale is therefore grad uated in volts. By varying the resistance the conditions in the instrument may be modified, so that it will measure from minute fractions of volts up to hundredths and thousandths.