The current transformer is composed of a closed core and windings. The voltage transformer is a transformer with an iron core. The current transformer can be short-circuited for the second time, but not open circuit; the voltage transformer can be opened for the second time, but not short-circuited. The winding with more turns of the current transformer and the thin wire diameter is connected to the measuring instrument, while the winding with the few turns of the voltage transformer and the thick wire diameter is connected to the measuring instrument. The current transformer is a winding with a small number of turns and a thick wire diameter is used as a primary winding to connect in series with the circuit to be measured, while a voltage transformer is a winding with a large number of turns and a thin wire diameter that is connected in parallel with the circuit to be measured as a primary winding . Compared with the load on the secondary side, the primary internal impedance of the voltage transformer is so small that it can be ignored. It can be considered that the voltage transformer is a voltage source; while the primary internal resistance of the current transformer is so large that it can be considered a A current source with infinite internal resistance.