CAUTION: with the Simpson meter, as with all analog meters, care must be taken to put the meter in the circuit with the proper polarity and on the proper range, or the meter can easily be damaged. Current must flow into the meter terminal which on analog meters such as this is usually labeled "+" or "V-A" or some such thing and is often colored red, and out of the terminal which is usually labeled "COM" or "GROUND" or "-", etc. and is often colored black. Ammeters should always be set initially to the least sensitive scale (labeled with the largest values of current) and then turned to more sensitive ranges until a good needle deflection is obtained. Similar precautions hold when using the Simpson as a voltmeter; polarity must be observed and you should start on the least sensitive scale, then switch to more sensitive ranges to get a good reading. These precautions are largely unnecessary for our digital meters multimeter (DMM), which if used wrong merely announces that fact by an overload indication.
Taking these precautions, set up the circuit below. Throughout this part, leave the DMM in the circuit, operating as a current meter.
Figure 1.
Put the Simpson in the circuit as a current meter (in series with the DMM) and adjust the DC voltage supply until the current is 1 mA. Do the DMM and the Simpson agree? Then remove the Simpson from the circuit. Does the current (as measured by the DMM) change as a result of removing the Simpson? Now use the Simpson as a voltmeter to measure the voltage across the resistor. Pay close attention to the current (measured on the DMM): does the current change when the Simpson is attached to measure the voltage? Last, disconnect the resistor from the circuit and use the Simpson to measure its resistance. Do the readings of V, I, and R verify Ohm's Law? Record the measurements and the percent error observed in V = I R, with readings taken on the Simpson. Measure the same three quantities with the DMM and calculate the error in V = I*R again. Comment on your observations.
A meter is said to "load" a circuit if attaching it changes the voltages or currents in the circuit being measured. In principle this loading should be zero. Set up the circuit below. Attach the DMM to measure V1.
Figure 2.
Leaving the DMM attached, connect the Simpson to measure V1 also. Does the addition of the Simpson affect the circuit? Record your observations.
The degree of loading by the Simpson can be calculated. Its effect on the circuit is the same as if a resistor were attached, with a value of "20,000 ohms per volt" where the "volt" refers to the range used. For example, on the 10 volt scale it affects the circuit like connecting a resistor of 200K (= 20K ohms/volt x 10 volt scale.) Calculate the expected loading, that is, how much you would expect V1 to change when the Simpson is attached. Compare this with your observations.
Now repeat this experiment for V2. Explain why the loading is less in this case.