ELECTRICAL MEASURING INSTRUMENTS

the name given to instruments which measure electric power, energy, voltage, or current. The majority of such instruments are current-operated. Thus, with the exception of electrostatic voltmeters, al} voltmeters are really current measuring instruments; but since this current is made to be proportional to the P.D. between the voltmeter terminals, the scale reading is proportional to the voltage being measured. One and the same instrument may be used as an ammeter or as a voltmeter, by providing it with shunts for use as an ammeter, and series resistances for use as a voltmeter. If the current to be measured is large, the shunt will have a very low resistance compared with that of the instrument, so that only a small fraction of the total current. pa es through the instrument. Similarly, when a large P.D. is being measured, the series resistance wilh have a very high value, so that the current through the instrument may not exceed that which gives full-scale reading. By using shunts or series resistances of different values, different ranges can be given to the instruments. In addition to the types already described (see AMMETER), there is a cla Electric Fish depending on the mutual action of current-carrying cenductors placed near one another. This type is largely used in alternating-current work. It is also specially suitable for power measurements, and practically all wattmeters work en this principle. The Siemens Dynamometer was the first instrument of this type. The Kelvin Standard Balance is a special form of dynamometer, in which the mechanical turning moment due to weights on a beam is balanced by the electrical turning moment due to currents in fixed coils and in coils attached to the ends of 99 ELECTRIC BATTERY the beam. The electrostatic voltmeter mentioned above is the only instrument which is operated by a P.D. instead of a current. In ita moving vane is attracted into a fixed pair of quadrants; or a set of vanes is attracted into a set of quadrant cells. The majority of electricity meters are of the motor type, i.e. a disc is driven by motor action at a speed which is proportional to the power pa ing through the meter. The disc spindle engages with gearing hich drives the pointers on a set of dials recording the energy units. There are also meters depending on electrolytic action (Wright meter); or on the difference in period between two pendulums, one of which is controlled by the load current (Aron meter).