1. Basic Measuring Instruments

2. * There are 3 types of instruments we use in 50A: * Ammeter * Voltmeter * Ohmmeter

3. * To understand loading effects * To understand a practical use for series and parallel circuits * To what you are doing when you use them

5. Ammeter

9. * When you turn the knob to change scales on an ammeter, you are changing the shunt resistor. Ammeter

12. * AN IDEAL AMMETER HAS 0 OHMS OF INTERNAL RESISTANCE!! * Since no ammeter is perfect, the result is called loading effects.

13. * Which circuit would be more effected by the loading effects of the ammeter if the total internal resistance of the ammeter is 1Ω? * Why is this not a completely fair question? * Because the scale would have to be changed for each circuit. The FSD of the first circuit would be 1A and the FSD of the second circuit would be 1mA. * This means the internal resistance of the ammeter for the first circuit would be.01Ω, and for the second circuit it would be 10Ω. * So for the first circuit the total resistance would be 20.01Ω and for the second circuit, the total resistance would be 20010Ω. Same % error.

14. * To Review Ammeters: * The larger the Full Scale Deflection or range setting, the smaller the internal resistance of the ammeter. * When measuring current, the ammeter affects or adds resistance to the circuit since the ammeter is placed in series. This causes the total resistance to be larger, which causes the total current to be less than it actually should be.

16. Voltmeter What about Voltmeters?

17. Voltmeter

19. Multi-range Voltmeter

20. * AN IDEAL VOLTMETER HAS ∞ OHMS OF INTERNAL RESISTANCE!! * Since no voltmeter is perfect, the result is called loading effects.

21. Multi-range Voltmeter Voltmeter sensitivity is expressed in ohms/volt (Ω/V). A voltmeter is considered more sensitive if it draws less current from the circuit. The sensitivity of a voltmeter that draws 50uA is the inverse of this, 20,000 ohm per volt.

22. Multi-range Voltmeter

23. * The internal resistance of our DMM is always 10MΩ no matter range is selected. (That’s because there is no range to select.)

24. 51.3V 48.7V 95.2kΩ

25. What about Ohmmeters? Ammeter

26. What about Ohmmeters? Ammeter

27. What about Ohmmeters? Ammeter