Play video 1 with potentiometer. Experimental purpose: 1. Master the working principle of UJ36a potentiometer; 2. Master the basic method of measuring with UJ36a box potentiometer. 2. Experimental instruments and equipment: 1 UJ36a potentiometer, 1 DC regulated power supply, 1 fixed value resistor, 1 milliammeter, 1 resistor box, 1 key, wires, etc. Experimental principle: 1. The measurement process of compensation method is the process of testing the real parameters of the system through experimental instruments, and the corresponding experimental methods and testing means should be based on the principle of not changing (or trying not to change) the original state of the tested system. However, this principle is often difficult to realize in many measurement processes. For example, when the voltmeter is connected across the terminals of Figure 7- 1 and Figure 7-2 of the tested power supply, the reading of the voltmeter is not the electromotive force ex of the tested power supply, but the terminal voltage u, as shown in Figure 7- 1. According to the ohm's law of the whole circuit, there is u = ex-I? 6? 7 r where r is the internal resistance of the resistance to be measured. The reason for this result is that the connection of voltmeter changes the original state of the circuit under test. In order to accurately measure the electromotive force of the power supply, it can be measured according to the circuit in Figure 7-2. EX in the figure is the power supply to be tested; E0 is an adjustable standard power supply; G is galvanometer. If E0 is adjusted to make the galvanometer display zero, the electromotive forces of the two power supplies in the loop are equal in magnitude and opposite in direction. At this time, we call the circuit a compensation circuit. Under the compensation condition, if the size of E0 is known, Ex can be found. According to this measurement principle, the method of measuring the measured function is called compensation method under the condition of providing an adjustable standard quantity to offset the measured function. 2. Working principle of 2.UJ36A box potentiometer: Figure 7-3 Measuring voltage or electromotive force with potential difference is a typical example of compensation method. As shown in Figure 7-3, E is the working power supply, EN is the standard battery electromotive force, Ex is the electromotive force or voltage to be measured, G is the transistor amplification galvanometer, RP is the working current adjustment resistor, R is the electromotive force compensation resistor to be measured, RN is the standard battery electromotive force compensation resistor, and K is the transfer switch. Adjust the working current first when measuring. Turn k to the "standard" position and adjust Rp to make the galvanometer point to zero. At this time, the electromotive force of the standard battery is compensated by the voltage drop across the resistor RN. So EN = I? 6? 7 RN (1) where I is the current flowing through RN and r, which is called the working current of the potentiometer. After adjusting the working current, move the K plate to the "unknown" position, and at the same time move the Q contact to make the galvanometer point to zero again. At this time, the reading of contact Q on R is RQ, which means that the measured electromotive force or voltage is compensated by the voltage drop across RQ resistor. So Ex = I? 6? 7 RQ (2) Divide formula (2) and formula (1) to get formula (3). It is not difficult to see from Formula (3) that the measured electromotive force or voltage can be obtained by measuring the ratio of RQ to RN and multiplying it by the electromotive force EN of the standard battery. When the circuit is completely compensated, no current flows through the measured loop, and the measuring loop does not absorb power from the measured loop, so the measurement accuracy is very high. 3. Principle of voltage measurement by potentiometer: As shown in Figure 7-4, when the K key is turned on, a current I will flow through the constant resistor R, and then a certain voltage Ux will be generated at both ends of R, which can be measured by potentiometer. When the resistance value of the resistor box Rf changes, the current I flowing through the fixed-value resistor R and the voltage UX across the fixed-value resistor R will also change accordingly. Figure 7-4 IV. Experimental steps: 1. Connect the circuit according to Figure 7-4, and adjust the regulated power supply to 2V; 2. Adjust the resistance box Rf, and adjust the working current I to the specified value in Table 7- 1; 3. Connect the voltage to be measured to the "unknown" terminal of the potentiometer, and be careful not to connect the positive and negative poles wrong; 4. Turn the proportional switch to the position of "×0.2" or "× 1", and after 3 minutes, adjust the "zero adjustment" knob on the panel to make the vibrating mirror point to zero (the first zero adjustment); 5. Turn the toggle switch to the "standard" end, and adjust the potentiometer RP to make the galvanometer point to zero (secondary zero adjustment); 6. Turn the toggle switch to the "unknown" end, adjust the stepping disc and sliding disc, and make the vibrating mirror zero again (the third zero adjustment); 7. The voltage or electromotive force Ux to be measured is calculated according to the following formula: Ux = (step dial reading+scribing dial reading) × magnification 8. Change the working current and do it for 5 times. 5. experimental record: table 7- 1 multiplied by123456i/ma20304050100150ux/mv× 0.2: 0 ~ 46mv, precision10μ v..