Papers to write network: two TESTING RESISTANCE "ammeter connected, in high school physics textbooks about much, only appear in the exercises, it is very important it not only contains the Ohm's law and circuit knowledge of the connections, etc., as well as helping to train students in the experimental system error analysis students often feel this part of knowledge principle straightforward, but often at a loss to do title, did not understand the knowledge thorough, no methodology timely finishing and Summary of I own teaching practice, students are now analyzed and summarized as follows:
First, to clear the system error.
To understand the measured values ​​in the two configurations exactly which part of the resistance of the circuit. Someone will say: of course, the resistance is to be measured. Fact that "voltmeter as open circuit, ammeter seen as short-circuit results in the ideal case. voltmeter resistance is not infinite, ammeter internal resistance is not zero, precisely because of this, the two connected will cause errors. shown below:

Figure A ammeter external diversion effect of the voltmeter, ammeter reading is the voltmeter and the measured resistance in parallel circuit the total current, of course, the readings of the voltmeter voltmeter and tested resistance in parallel circuit voltage, so an external law The measured value is the total resistance of the voltmeter and in parallel to the resistor under test, i.e., R the measured = RV and Rx. Obviously, when Rx << RV smaller error.

Figure B ammeter connection, partial pressure of ammeter, voltmeter reading ammeter and measured resistance in series circuit voltage, of course, the ammeter reading ammeter and measured resistance in series circuit current internal method The measured value is the total resistance R measured voltage meter and the measured resistance in series = RA strings Rx. Obviously, the error is smaller when the Rx >> RA.

Second, in a different case, select the appropriate connection to reduce systematic errors.
(1 equipment if a voltmeter and an ammeter, and the measured resistance ammeter internal resistance all know about the numerical case, then select the appropriate connection, in order to reduce systematic errors. Rx << RV, external law If Rx >> RA, within the connection.

(2 equipment if a voltmeter and an ammeter, but the measured resistance ammeter internal resistance and did not know about numerical case, the available test in conflict with the law, that has two connected. If ammeter reading did not change significantly, then the voltmeter shunt induced error is small, appropriate external law, if the voltmeter readings did not change significantly, the ammeter partial pressure caused the error is small, then the connection is appropriate within.

(3 equipment if a voltmeter and an ammeter, and a meter internal resistance of the known cases, may wish to use (1 method, because it can eliminate the system error If the internal resistance of the voltmeter is known, in an external law and resistance RV and Rx, can be obtained accurate Rx voltmeter shunt induced error can be eliminated, so the selection of an external law known ammeter internal resistance, including access law and resistance RA string Rx, you can find the accurate Rx eliminate error caused by the partial pressure of that ammeter, so the selection of internal law.

(4 equipment if the two voltmeter or two current meter, and an ammeter internal resistance is known accurately value, the table can be used as an addition table such as to two voltage meter, voltmeter internal resistance is known readings in addition to its internal resistance, that is, the current through it, so it can be used as ammeter with; such as to the two current meter, ammeter readings multiplied by its internal resistance internal resistance is known, that is, the voltage across it, so it can be used as a voltmeter with, so choose the ammeter within the same external method (Links to free papers Download Center http://eng.hi138.com

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