The next type of current measurement is using current transformers. These are fairly inexpensive, but it is necessary to break the circuit and install a loop around the magnetic core of the current transformer. The current transformer will only measure AC current. The current transformer has been used as a transducer for years. As when used to transform voltages, a transformer will induce a current in the secondary winding which is equal to the current passing through the primary divided by the turns ratio. The primary usually consists of a single turn, formed by the current carrying conductor physically passing through the core. The secondary will contain several turns, to reduce the current to a level which can be easily measured. To convert the secondary current into a voltage which can be measured by the oscilloscope, a termination resistance is used. Often the termination resistor is built into the current transformer. Current transformers are accurate over a specified bandwidth, both high frequency and low frequency. The largest drawback is the inability to measure DC. In fact, a DC component will add a static flux in the core. This shifts the operating point on the B-H curve, which moves the center of the dynamic range for AC components. If the DC component is large enough, the core may saturate, completely blocking the ability to measure the AC component of the waveform. Like the shunt, a current transformer requires that the test conductor be opened and physically passed through the core. Current transformers add impedance containing both a resistive and inductive (reactive) element to the test circuit.