BASIC METERS

portion of it) directly to the meter terminals. The resulting excessive current burns up the meter coil andrenders the meter useless until repaired.Q-11. In what manner are current-measuring instruments connected to a circuit?If you do not know the approximate value of current in the circuit, you should take a reading at thehighest range of the ammeter; then you should switch progressively to lower ranges until a suitablereading is obtained. Most ammeter scales indicate the current being measured in increasing values fromleft to right. If you connect the meter without observing proper polarity, the pointer may be deflectedbackwards (from right to left). This action often damages the meter movement. You should ensure thatthe ammeter is always connected so that the current will flow into the negative terminal and out thepositive terminal. Figure 3-6 shows various circuit arrangements and the proper ammeter connectionmethods to measure current in various portions of the circuit.Figure 3-6.—Proper ammeter connection.Q-12. An ammeter should always be connected so that current will flow into what terminal and out ofwhat terminal?Ammeter SensitivityAmmeter sensitivity is determined by the amount of current required by the meter coil to producefull-scale deflection of the pointer. The smaller the amount of current required to produce this deflection,the greater the sensitivity of the meter. A meter movement that requires only 100 microamperes for fullscaledeflection has a greater sensitivity than a meter movement that requires 1 milliampere for the samedeflection.Q-13. (True or False) The larger the current required to produce full-scale deflection of the meter coil,the better the sensitivity of the meter.Good sensitivity is especially important in ammeters to be used in circuits in which small currentsflow. As the meter is connected in series with the load, the current flows through the meter. If the internalresistance of the meter is a large portion of the load resistance, an effect known as METER-LOADINGwill occur. Meter-loading is the condition that exists when the insertion of a meter into a circuit changesthe operation of that circuit. This condition is not desirable. The purpose of inserting a meter into a circuitis to allow the measurement of circuit current in the normal operating condition. If the meter changes the3-11