Twist-on wire connector
In the UK, they were made out of ceramic and sold under the brand "Scruit."
Twist-on wire connectors are available in a variety of sizes and shapes and are typically made from plastic, with a tapered, conducting coiled metal insert that threads onto the wires to hold the wires secure. When the connector is twisted onto the stripped ends of the wires, they are drawn into the metal insert. The electrical continuity is maintained by both the direct wire to wire contact and by the metal insert. Twist-on wire connectors are intended to be installed by hand only, and may include molded grooves or blades intended to assist in hand installation. Hex tips or hex-like wings are commonly molded into higher-quality connectors that allow them to be installed with a common nut driver.
Twist-on wire connectors are commonly color-coded to indicate the nut's size (and hence, its wire capacity). They are commonly used as an alternative to terminal blocks or soldering conductors together because they are faster and allow subsequent removal for future rework.
Twist-on wire connectors are not generally recommended for use with aluminum wire; however, several companies manufacture twist-on connectors designed specifically for use with aluminum conductors. Also, most connectors are not rated for wet use (such as exposed outdoors); a special gel-filled connector must be used in this circumstance.