Potentiometers are widely used in chemical experiments, mainly for measuring electromotive force and calibrating various ammeters. Common domestic potentiometers are student type, 701 type, UJ-l type, UJ-2l type, UJ_25 type, UJ_34 type, UJ_9 type, etc. Some of them are low-resistance potentiometers, while UJ-2l, UJ_25, UJ_34, and UJ_9 are high-resistance DC potentiometers. Different types of potentiometers can be selected according to the different systems to be measured. Generally, a high-resistance potentiometer is used for a high-resistance system, and a low-resistance potentiometer is used for a low-resistance system.
1. Working principle
The potentiometer is a balanced voltage measuring instrument designed according to the measurement principle of the compensation method (also known as the cancellation method).
The electromotive force of a battery cannot be measured directly with a voltmeter. Because the battery is connected to the voltmeter and a current passes through it, the electrode polarization will occur on the electrode, and the electrode will deviate from the equilibrium state as a result. In addition, the battery itself has internal resistance, so the voltmeter
Only the terminal voltage of the irreversible battery is measured. Measuring the electromotive force of the battery can only be carried out when there is no current passing through, and the compensation method (or cancellation method) is required to measure the electromotive force of the battery.
The measurement principle of the cancellation method is to connect an applied potential difference of equal size and opposite direction in parallel on the battery to be tested, so that no current flows through the battery to be tested, and the magnitude of the applied electromotive force is equal to the electromotive force of the battery to be tested.
2. How to use the potentiometer
The board schematic diagram of the UJ-21 high-resistance DC potentiometer is shown in Figure II_27.
l. Preparation
Before using the potentiometer, put the reversing switch ⑰ in the "off" position, and release all the 3 thick, thin and short buttons ⑭, ⑮, ⑯ on the lower left. Connect the galvanometer, working battery, standard battery and the pool to be tested according to the corresponding positions in Figure 1I-27. When connecting, except for the galvanometer, which does not distinguish between positive and negative poles, other batteries have positive and negative poles. When connecting, pay attention to the wrong positions of the positive and negative poles.
2. Adjust the working current
First, adjust the standard battery temperature compensation knobs ① and ② to the electromotive force value of the standard battery at the experimental temperature; then turn the reversing switch 〇 to the "standard" position, press the "coarse adjustment" button, and the galvanometer When the light spot deflects, release the button immediately. Adjust the variable resistor R from large to small according to the light spot deflection direction (that is, from knob ③ to knob ⑦), repeatedly press the "coarse adjustment" button and adjust R until the detection If the deflection of the light spot of the galvanometer is not obvious, press the "fine adjustment" button again to adjust the variable resistor R until the light spot of the galvanometer does not deflect. At this time, the working current is a certain value.
3. Measure the electricity or momentum of the battery under test
Turn the reversing switch 〇 to the "unknown" position, first press the "coarse adjustment" button, and adjust the measurement knobs ⑧~⑬ in turn, so that the deflection of the light spot of the galvanometer is not obvious, and then press the "fine adjustment" button to adjust the measurement Knob until the light spot of the galvanometer does not deflect. Read the digits of the electromotive force of the battery to be tested from the reading window corresponding to the ⑧~⑬ measurement knob.
If during the measurement process, the light spot of the galvanometer keeps deflecting in one direction, and the balance point cannot be found, it may be caused by the following reasons: ①The positive and negative poles of the battery are connected incorrectly; ②The circuit is in poor contact; Open circuit; ④ working battery voltage is not enough. It should be checked and re-determined after troubleshooting.
