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 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.
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 circuit operation and changes the amount of current flow, the reading you obtain will be in error. An example of this is shown in figure 3-7.
Figure 3-7. - Ammeter loading effect.
In view A of figure 3-7, the circuit to be tested has an applied voltage of 100 millivolts and a resistance of 100 ohms. The current normally flowing in this circuit is 1 milliampere. In view B, an ammeter that requires 1 milliampere for full-scale deflection and that has an internal resistance of 100 ohms has been inserted. Since 1 milliampere of current flow is shown in view A, you might naturally assume that with the meter inserted into the circuit, a full-scale deflection will occur. You might also assume that the 1 milliampere of circuit current will be measured. However, neither of these assumptions is correct. With the ammeter inserted into the circuit, as shown in view B, the total resistance of the circuit is 200 ohms. With an applied voltage of 100 millivolts, applying Ohm's law shows the actual current (Icircuit) to be 0.5 milliampere.
Since the meter reads 0.5 milliampere instead of the normal value of current, the meter reveals that a definite loading effect has taken place. In cases such as this, the use of ammeters, which have a lower internal resistance and a better current sensitivity, is desirable.
Up to this point, we have been discussing the 100-microampere D'Arsonval movement and its use as an ammeter. However, it can also be used to measure voltage if a MULTIPLIER (high resistance) is placed in series with the moving coil of the meter. For low-voltage instruments, this resistance is physically mounted inside the meter case with the D'Arsonval movement. The series resistance is constructed of a wire-wound resistance that has a low temperature coefficient wound on either a spool or card frame. For high-voltage ranges, the series resistance can be connected externally. A simplified diagram of a voltmeter is shown in figure 3-8.
Figure 3-8. - Internal construction and circuit of a simplified voltmeter.