Learning to use a Multimeter

Today, my father taught me how to use a multimeter . He gave me a digital multimeter of the FLUKE 15B model. It‘s a really nice-looking device. This multimeter is auto-ranging, so there’s no need to set the measurement range. We need to insert the red test lead into the VΩ hole (when measuring voltage and resistance) , the mA hole (when measuring small current) , or the 10A hole (when measuring large current), and insert the black test lead into the COM hole before using it.

Measuring voltage

Set the multimeter to the voltage measurement range (DC voltage range is V⎓, and AC voltage range is V~). Connect the probes to the two ends of the measured power supply. For DC voltage, pay attention to the positive and negative polarities. The red test lead should be connected to the positive pole, and the black test lead should be connected to the negative pole. Read the voltage value displayed on the multimeter .

Measuring resistance

Set the multimeter to the resistance measurement range (Ω).

Firstly, short-circuit the two test leads and adjust the zero adjustment knob to make the multimeter display 0Ω. Then, touch the two test leads to the two ends of the measured resistance respectively, and read the resistance value displayed on the multimeter.

Measuring Body’s resistance

Interestingly, we can use a multimeter to measure the resistance of the human body. For example, the resistance between my two hands is approximately 2MΩ. I measured the resistance of myself, my father, and my mother, and the values were different for each of us.

According to what is stated in the physics textbook, the safe voltage for the human body is below 36 volts. Then, what should be the maximum value that the safe current should not exceed?

It should be noted that based on Ohm's law I\=U/R (where I represents current, U represents voltage, and R represents resistance). Given that the safe voltage for the human body is 36V, and as mentioned before, the resistance between the two hands of the human body is approximately R\=2MΩ=2×106Ω, then the safe current I\=U/R\=36÷(2×106)=1.8×10−5A\=18μA.

However, it should be pointed out that the resistance of the human body is not a fixed value. It will vary due to many factors such as the dryness or wetness of the skin and the contact area. Generally speaking, the resistance of the human body is relatively large when the skin is dry, while it will decrease significantly when the skin is wet. Therefore, in practice, the value of the safe current is more conservative. Usually, it is considered that the safe current for the human body should not exceed 10 mA.

One wrong way of measuring resistance is not to hold both ends of the resistor and the test leads with your fingers simultaneously. Since the human body's resistance is connected in parallel [in this situation], the resistance reading will be smaller than the actual value.

Measuring current

Set the multimeter to the current measurement range (DC current range is A-, and AC current range is A~).

Disconnect the circuit and connect the multimeter in series to the measured circuit. For DC current, also pay attention to the direction of the current. The red test lead should be connected to the end where the current flows in, and the black test lead should be connected to the end where the current flows out.

Turn on the circuit and read the current value displayed on the multimeter. As shown in the figure, the voltage is 5 volts. When a resistor is connected, the ammeter (multimeter set to measure current) shows that the current is 62 milliamperes. The resistance can be calculated to be 81 Ω.