DC measurements: voltage vs. current
It is generally preferred to make voltage measurements rather than current measurements because, when set up properly, voltage measurements are safer for the equipment being measured. This is particularly true when the meter is physically far away from the measurement point, forcing you to use long test leads.
The things to think about when making measurements with long test leads (generally greater than 6 feet) are:
Lead resistance, which affects all frequencies;
Transmission line effects, which include lead inductance, and starts showing up at high audio frequencies; and
Electromagnetic interference (EMI), which appears in the extremely low frequency (ELF) band below 30 Hz.
The basic dc measurement circuit consists of an excitation power source and three resistances: sensor output resistance, transmission line (test lead) resistance, and meter resistance. The only thing electrically connecting these elements is the circulating current. When you make a measurement, what you are really measuring is the voltage drop across the meter resistance due to the circulating current.
Voltage sources have a low resistance, with an “ideal” source defined as one with zero resistance. A thermocouple, for example, has a Thevenin equivalent circuit comprising the excitation source and sensor resistance, with the source producing a voltage in the millivolt range that is proportional to the hot/cold junction temperature difference, and a resistance well below an ohm. It is the excitation source, therefore, that controls the circulating current.
A thermister, on the other hand, requires an outside excitation source, with the transducer element being the sensor resistance on the order of a hundred ohms. Its Thevenin equivalent still comprises the excitation source and sensor resistance. Their roles, however, differ. The sensor resistance now controls the circulating current.
Typical test leads are made of #22 copper wire, which has a resistance of 0.019 ohms/ft. Test leads using 2 #22 wires, 6 ft long thus have a total resistance of 0.228 ohms. That’s tiny compared to the thermister’s resistance, but significant compared to the thermocouple’s resistance. Lead resistance, however, can make a big difference (approximately 2%) to the thermocouple measurement if the transmission line distance to the sensor grows to, say, 60 ft. (The transmission line distance is the distance the signal must travel along the lead-wire pair as routed to the meter.)
When it comes to meter resistance, always use a high impedance meter for voltage measurements and a low impedance meter for current measurements. This translates into having a meter resistance at the opposite end of the scale from your source resistance. Whether making current or voltage measurements, you always want the lead (transmission line) resistance to be relatively small compared to the largest resistance among the other components.
Digital multimeters have input impedances at least on the order of 100 k ohms, and oscilloscope resistances run a couple of orders of magnitude higher. Using such instruments, lead resistance for even very long leads (hundreds of meters) disappears.
If you try to use a high-impedance meter for a thermocouple measurement, however, the meter resistance controls the circulating current. The meter will read the excitation voltage no matter what the temperature is. You have to treat the thermocouple measurement as a current measurement, even though the sensor resistance is only around 100 ohms. That means using a very low resistance meter, and paying close attention to lead resistance.
That explains why it is preferrable to use voltage measurements for all long-range applications. Properly set up, lead length largely becomes irrelevant. So if, for example, you want to measure the current through a dc motor from a separate control room, you need to find a way to change it from a current measurement to a voltage measurement.