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Laboratory 4: Instrument Characteristics

20108

Laboratory #4:

Instrument Characteristics

Objectives:

Understand the effect of introducing a measuring device into a circuit, and how the instrument

affects the measurement.

Introduction:

One well known understanding from quantum mechanics is that the act of looking at (e.g., meas-

uring) any system changes the system’s behavior. That is certainly true for electrical and elec-

tronic systems. Thus, it is critically important for engineers to understand the characteristics of

measurement equipment — how it works and how it interacts with the sample being measured

— in order to make accurate and useful measurements on systems, especially in unusual circum-

stances. This laboratory seeks to provide such understanding and knowledge for two of the most

important tools for electrical measurements: the ammeter and the voltmeter.

Pre-lab Preparation:

1. Review Appendix A: Safety.

2. Review Appendix B: Equipment and Instrument Circuits through Digital Multimeters.

3. Study the textbook’s descriptions of the Voltmeter and the Ammeter.

4. Review Ohm’s Law, Kirchhoff’s Voltage Law, and Kirchhoff’s Current Law.

Safety Precautions:

1. Always turn off power to the circuit when changing the circuit.

2. Only reapply power after verifying that the circuit is properly wired and that the voltage

to be applied is at or below the required value.

3. Failure to turn off power when making circuit changes is a major reason for blowing

fuses in the equipment, thereby rendering the equipment unusable and wasting your time

and that of others. Please carefully check circuit wiring, resistor settings, and voltage set-

tings before applying power to the circuits.

Equipment Needed:

 DC Power supply

 Digital multimeter

 Resistance decade boxes (2)

Procedure:

Ammeter

1. Determining the internal resistance of the digital ammeter.

For the following circuit, set the range of the ammeter to the 2mA scale or the closest setting

to this, and do not adjust this scale at any time during the procedure. First, determine the cur-

rent ‘I’ in the following circuit without the variable resistance in the circuit. Then connect the

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