Electromagnet and modem: what they have in common

1. Step 1. Electromagnet — is part of — Intercom. An electromagnetic lock.
2. Step 2. Intercom — is a kind of — Telephone. An intercom is a simple telephone between the door and the flat.
3. Step 3. Telephone — is used in — Modem. It works over a telephone line.

Open Evothings