A transformer that is used in conjunction with a measuring instrument. It utilizes the current-transformation (current transformer) or voltage-transformation (voltage transformer) properties of a transformer. The primary winding forms part of the main circuit and carries the current or voltage to be measured. The secondary winding is connected to the measuring instrument, such as an ammeter or voltmeter. Instrument transformers are used to extend the dynamic range of a.c. instruments and to isolate the instruments from circuits operating at high voltages.