Electron gun and programmable logic controller: what they have in common

1. Step 1. Electron gun — is part of — Electron microscope. A source of electrons.
2. Step 2. Electron microscope — uses — Electromagnet. Magnetic lenses.
3. Step 3. Electromagnet — is part of — Relay. An electromagnet switches the contacts.
4. Step 4. Relay — evolved into — Programmable logic controller. It replaced relay cabinets.

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