You generally need a transformer to operate a doorbell because doorbells are designed to work with much smaller voltages than what you typically get from wiring in a house. One particular doorbell needs a potential difference of 16V to work, while the standard outlet voltage (in the United States) is 120V.

This particular doorbell is also designed to draw a current of 2.1 A. Assuming 100% efficiency, what will be the current in the primary coil?