Rotary transformer

Slip rings can be used for the same purpose,[citation needed] but are subject to friction, wear, intermittent contact, and limitations on the rotational speed that can be accommodated without damage.

Wear can be eliminated by using a pool of liquid mercury or liquid metal alloy instead of a solid ring contact, but the toxicity and slow corrosion of mercury are problematic, and very high rotational speeds are again difficult to achieve.

Rotary transformers are constructed by winding the primary and secondary windings into separate halves of a cup core; these concentric halves face each other, with each half mounted to one of the rotating parts.

In brushless synchros, typical rotary transformers (in pairs) provide longer life than slip rings.

The supporting electronics, including the tape heads or torque sensors, must be designed to accommodate this.

Cross-section diagram of a simple rotary transformer.
Prior to the development of the rotary transformer, a slip-ring pickup was used, though this was prone to developing signal noise due to corrosion of the slip rings.