3-11
portion of it) directly to the meter terminals. The resulting excessive current burns up the meter coil and
renders 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 the
highest range of the ammeter; then you should switch progressively to lower ranges until a suitable
reading is obtained. Most ammeter scales indicate the current being measured in increasing values from
left to right. If you connect the meter without observing proper polarity, the pointer may be deflected
backwards (from right to left). This action often damages the meter movement. You should ensure that
the ammeter is always connected so that the current will flow into the negative terminal and out the
positive terminal. Figure 3-6 shows various circuit arrangements and the proper ammeter connection
methods 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 of
what terminal?
Ammeter Sensitivity
Ammeter sensitivity is determined by the amount of current required by the meter coil to produce
full-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 full-
scale deflection has a greater sensitivity than a meter movement that requires 1 milliampere for the same
deflection.
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 currents
flow. As the meter is connected in series with the load, the current flows through the meter. If the internal
resistance of the meter is a large portion of the load resistance, an effect known as METER-LOADING
will occur. Meter-loading is the condition that exists when the insertion of a meter into a circuit changes
the operation of that circuit. This condition is not desirable. The purpose of inserting a meter into a circuit
is to allow the measurement of circuit current in the normal operating condition. If the meter changes the
