Ammeter is based on the effect of the magnetic field strength of the current-carrying conductor in a magnetic field and made. Meter inside a permanent magnet, the magnetic field generated between the poles, a coil spring at each end of the spring coil in a magnetic field, an ammeter connected to the terminal spring, between the coil spring is connected by an axis, relative to the shaft ammeter on the front, there is a pointer. When a current is passed, the current along the spring, the shaft through a magnetic field, the magnetic induction current cut line, so the role of the magnetic field strength, deflection coil, drive shaft, pointer deflection. Since the magnetic force varies with the size of the current increases, thereby making it possible to observe the degree of deflection magnitude of the current through the pointer. Called magnetic-electric meter, is what we usually use the kind of laboratory. In the middle period, the use of meter range and is generally 0 ~ 0.6A 0 ~ 3A AC ammeter mainly uses electromagnetic system meter, electric meter and the Department of rectifier type meter measuring mechanism. Lowest range electromagnetic system measuring mechanism is about tens of milliamps, to improve the range, a pro rata reduction in the number of turns, and bold wires. When composing ammeter, dynamic and static parallel electric lines circle measuring mechanism, the minimum range of about tens of milliamps. To improve the range, to reduce the number of turns quiet and bold wire, or two still rings parallel to the series, the meter range will be doubled. Rectified AC current meter measurements available, only when the AC sinusoidal waveform, meter reading was correct. To expand the range can also be used shunt. Furthermore, the meter can also be measurement means for measuring the thermoelectric high frequency current. Large range AC ammeter used in power systems mostly use electromagnetic system ammeter 5A or 1A, and with appropriate current ratio of the current transformer. Ammeter and voltmeter measuring means is basically the same, but the connection of the measuring lines is different. Therefore, the selection and use of ammeter and voltmeter should pay attention to the following seven. Select ⒈ type. When the measurement is DC, the DC should be selected table, that magnetic lines measuring mechanism of the instrument. When measuring the exchange, should pay attention to the waveform and frequency. If a sine wave, RMS can be measured only in terms of other values ??(such as maximum, average, etc.), you can take any kind of exchange table; What if a non-sine wave, is to be measured should distinguish value, RMS or ferromagnetic magnetic system can be used to measure body electric lines and meters, the average rectified line measuring mechanism is chosen instrument. Electric lines used in measuring mechanism precision measurement instrument AC current and voltage. ⒉ selection accuracy. Due to the higher accuracy of the instrument, the price more expensive, maintenance is more difficult. Moreover, if the other conditions mismatch, no matter how high the level of accuracy of the instrument, it may not be able to get accurate results. Therefore, in the choice of a lower accuracy accurate measuring instruments to meet the requirements of the situation, do not use high accuracy instrumentation. Typically 0.1 and 0.2 meter as a standard table selection; 0.5 and 1.0 meter used as a laboratory measurements; 1.5 meter below the general selection as engineering survey. ⒊ range of choices. We should give full play to the role of the instrument accuracy must also be measured according to the size limit of the amount of a reasonable selection of instruments, such as the inappropriate choice of the measurement error will be immense. Usually the instrument is measured on the instrument indicates the maximum range greater than 1/2 to 2/3 or more, but not more than the maximum range. ⒋ resistance selection. When selecting instruments, should be based on the size of the measured impedance to select the instrument resistance, otherwise it will bring greater measurement error. Due to the size of the internal resistance of the power consumption reflects the instrument itself, so when measuring current, resistance should be chosen as small as possible ammeter; When measuring voltage, resistance should be chosen as large as possible voltmeter.