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把电流表改装成安培表和伏特表,需要知道电流表的三个参数,即满偏电流I_g、满偏电压U_g和内电阻r_g。其中内电阻r_g可以用电流表半偏法测出。测量原理如图1所示,R、R′分别为电位器和电阻箱。测量方法是先合上K_1,调整电位器R,使电流表指针达到满偏;然后再合上K_2,调节电阻箱R′,使电流表指针达到半偏,此时电阻箱的阻值R′等于电流计的电阻r_g。这是因为,电阻器与电流表并联后,通过电流表及电阻箱的电流强度均为1/2I_g,且1/2I_gR′=1/2I_gr_g。所以,r_g=R′。 (1) I_g为电流表满偏电流并假定通过电路的总电流不变。由于K_2合上的前后,电路中的总电流并非都是I_g,因而流过电阻箱的实际电流并不等于1/2I_g,并且需要读两次电表,这样就会造成方法上的误差。下面来分
To convert an ammeter into an ammeter and a voltmeter, one needs to know three parameters of the ammeter, that is, the full bias current I_g, the full bias voltage U_g, and the internal resistance r_g. The internal resistance r_g can be measured using the ammeter half-side method. The measurement principle is shown in Figure 1. R and R’ are potentiometers and resistor boxes, respectively. The measuring method is to close K_1 first, adjust the potentiometer R, and make the current meter pointer reach full deviation; then close K_2, adjust the resistance box R′ so that the ammeter pointer reaches semi-bias, and the resistance box’s resistance R′ equals the current. Resistor r_g. This is because, after the resistor is connected in parallel with the ammeter, the current intensity through the ammeter and the resistance box is 1/2I_g, and 1/2I_gR’=1/2I_gr_g. So, r_g=R’. (1) I_g is the ammeter full current and assumes that the total current through the circuit does not change. Since the total current in the circuit is not all I_g before and after K2 closes, the actual current flowing through the resistor box is not equal to 1/2I_g, and the meter needs to be read twice, which will cause a method error. The following points