What is the formula for connecting the red and black test pens? If it is the voltage and current range of a voltmeter, ammeter or multimeter, the red test lead is connected to the positive electrode, and the black test lead is connected to the negative electrode; red in and black out means that all currents are entered by the red test pen and flow out from the black test pen. If it is the electrical barrier known by the multimeter, that is, the black test lead is equivalent to the positive electrode of the battery, and the red test lead is equivalent to the negative electrode of the battery; because when measuring the resistance (there is no power supply outside), the current passing through the resistance is provided by the ohmmeter itself. At this time, for the ohmmeter, the black marker is connected to the positive pole of the internal power supply, and the red test pen is connected to the negative pole. If it is the voltage and current range of a voltmeter, ammeter or multimeter, the red test lead is connected to the positive electrode, and the black test lead is connected to the negative electrode; red in and black out means that all currents are entered by the red test pen and flow out from the black test pen. If it is the electrical barrier known by the multimeter, that is, the black test lead is equivalent to the positive electrode of the battery, and the red test lead is equivalent to the negative electrode of the battery; because when measuring the resistance (there is no power supply outside), the current passing through the resistance is provided by the ohmmeter itself. At this time, for the ohmmeter, the black marker is connected to the positive pole of the internal pow
